Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($)
$ in Thousands |
Mar. 31, 2016 |
Dec. 31, 2015 |
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 4,258,945
|
$ 4,279,347
|
| Equity securities |
$ 396,637
|
321,285
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 3,755,896
|
|
| Percentage of investment portfolio by credit rating agencies |
87.90%
|
|
| Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
88.20%
|
|
| Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 273,941
|
|
| Percentage of investment portfolio by credit rating agencies |
6.60%
|
|
| Investment Grade | BBB+ to BBB- Ratings | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
6.40%
|
|
| Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 229,108
|
|
| Percentage of investment portfolio by credit rating agencies |
5.50%
|
|
| Non-Investment Grade | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
5.40%
|
|
| U.S. Treasury Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 144,394
|
129,372
|
| U.S. Treasury Bonds | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| U.S. Treasury Bonds | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 144,394
|
|
| U.S. Treasury Bonds | Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Municipal Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 699,524
|
703,795
|
| Municipal Bonds | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Municipal Bonds | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 670,617
|
|
| Municipal Bonds | Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
95.90%
|
|
| Municipal Bonds | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 23,887
|
|
| Municipal Bonds | Investment Grade | BBB+ to BBB- Ratings | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
3.40%
|
|
| Municipal Bonds | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 5,020
|
|
| Municipal Bonds | Non-Investment Grade | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
0.70%
|
|
| Foreign Government Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 129,577
|
130,101
|
| Foreign Government Bonds | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Foreign Government Bonds | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 120,950
|
|
| Foreign Government Bonds | Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
93.30%
|
|
| Foreign Government Bonds | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 7,515
|
|
| Foreign Government Bonds | Investment Grade | BBB+ to BBB- Ratings | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
5.80%
|
|
| Foreign Government Bonds | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 1,112
|
|
| Foreign Government Bonds | Non-Investment Grade | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
0.90%
|
|
| Governmental Agency Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 354,941
|
418,091
|
| Governmental Agency Bonds | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Governmental Agency Bonds | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 354,941
|
|
| Governmental Agency Bonds | Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Governmental Agency Mortgage-Backed Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 2,044,958
|
2,055,673
|
| Governmental Agency Mortgage-Backed Securities | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Governmental Agency Mortgage-Backed Securities | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 2,044,958
|
|
| Governmental Agency Mortgage-Backed Securities | Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| U.S. Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 659,865
|
634,683
|
| U.S. Corporate Debt Securities | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| U.S. Corporate Debt Securities | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 296,764
|
|
| U.S. Corporate Debt Securities | Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
45.00%
|
|
| U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 182,015
|
|
| U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
27.60%
|
|
| U.S. Corporate Debt Securities | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 181,086
|
|
| U.S. Corporate Debt Securities | Non-Investment Grade | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
27.40%
|
|
| Foreign Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 225,686
|
207,632
|
| Foreign Corporate Debt Securities | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Foreign Corporate Debt Securities | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 123,272
|
|
| Foreign Corporate Debt Securities | Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
54.60%
|
|
| Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 60,524
|
|
| Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
26.80%
|
|
| Foreign Corporate Debt Securities | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 41,890
|
|
| Foreign Corporate Debt Securities | Non-Investment Grade | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
18.60%
|
|
| Preferred stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Equity securities |
$ 15,446
|
$ 15,467
|
| Preferred stocks | Equity securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
100.00%
|
|
| Preferred stocks | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Equity securities |
$ 8,869
|
|
| Preferred stocks | Investment Grade | BBB+ to BBB- Ratings | Equity securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
57.40%
|
|
| Preferred stocks | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Equity securities |
$ 6,577
|
|
| Preferred stocks | Non-Investment Grade | Equity securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
42.60%
|
|
| Available For Sale Securities Excluding Common Stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Total |
$ 4,274,391
|
|
| Available For Sale Securities Excluding Common Stocks | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Total |
3,755,896
|
|
| Available For Sale Securities Excluding Common Stocks | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Total |
282,810
|
|
| Available For Sale Securities Excluding Common Stocks | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Total |
$ 235,685
|
|