Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($)
$ in Thousands |
Jun. 30, 2016 |
Dec. 31, 2015 |
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 4,542,421
|
$ 4,279,347
|
| Equity securities |
$ 362,266
|
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 |
$ 3,974,644
|
|
| Percentage of investment portfolio by credit rating agencies |
87.20%
|
|
| Investment Grade | A- Rating or Higher | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
87.50%
|
|
| Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 313,774
|
|
| Percentage of investment portfolio by credit rating agencies |
7.10%
|
|
| Investment Grade | BBB+ to BBB- Ratings | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
6.90%
|
|
| Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 254,003
|
|
| Percentage of investment portfolio by credit rating agencies |
5.70%
|
|
| Non-Investment Grade | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
5.60%
|
|
| U.S. Treasury Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 124,388
|
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 |
$ 124,388
|
|
| 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 |
$ 766,915
|
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 |
$ 733,263
|
|
| 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 |
95.50%
|
|
| Municipal Bonds | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 27,446
|
|
| 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 |
3.50%
|
|
| Municipal Bonds | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 6,206
|
|
| Municipal Bonds | Non-Investment Grade | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
1.00%
|
|
| Foreign Government Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 143,317
|
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 |
$ 129,866
|
|
| 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 |
90.60%
|
|
| Foreign Government Bonds | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 9,930
|
|
| 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 |
6.90%
|
|
| Foreign Government Bonds | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 3,521
|
|
| Foreign Government Bonds | Non-Investment Grade | Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
2.50%
|
|
| Governmental Agency Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 341,755
|
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 |
$ 341,755
|
|
| 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 |
$ 2,217,812
|
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 |
$ 2,217,812
|
|
| 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 |
$ 718,382
|
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 |
$ 308,868
|
|
| 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 |
43.00%
|
|
| U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 202,157
|
|
| 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 |
28.10%
|
|
| U.S. Corporate Debt Securities | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 207,357
|
|
| 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 |
28.90%
|
|
| Foreign Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 229,852
|
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 |
$ 118,692
|
|
| 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 |
51.60%
|
|
| Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 74,241
|
|
| 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 |
32.30%
|
|
| Foreign Corporate Debt Securities | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt securities, Estimated fair value |
$ 36,919
|
|
| 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 |
16.10%
|
|
| Preferred stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Equity securities |
$ 15,169
|
$ 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 |
$ 8,866
|
|
| 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 |
58.40%
|
|
| Preferred stocks | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Equity securities |
$ 6,303
|
|
| Preferred stocks | Non-Investment Grade | Equity securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investment portfolio by credit rating agencies |
41.60%
|
|
| Available For Sale Securities Excluding Common Stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Total |
$ 4,557,590
|
|
| 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 |
3,974,644
|
|
| 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 |
322,640
|
|
| Available For Sale Securities Excluding Common Stocks | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Total |
$ 260,306
|
|