Debt and Equity Securities (Composition of Investment Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($)
$ in Thousands |
Mar. 31, 2016 |
Dec. 31, 2015 |
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 1,359,497
|
$ 2,499,093
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Available For Sale Securities Excluding Common Stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 1,219,083
|
|
| Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investments in unrealized loss position |
81.30%
|
|
| Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investments in unrealized loss position |
8.30%
|
|
| Investment Grade | Available For Sale Securities Excluding Common Stocks | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 990,592
|
|
| Investment Grade | Available For Sale Securities Excluding Common Stocks | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 101,450
|
|
| Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Percentage of investments in unrealized loss position |
10.40%
|
|
| Non-Investment Grade | Available For Sale Securities Excluding Common Stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 127,041
|
|
| Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 1,204,056
|
2,275,871
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | U.S. Treasury Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 37,870
|
107,355
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Municipal Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 74,614
|
146,655
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Foreign Government Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 14,245
|
13,868
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Governmental Agency Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 37,002
|
209,272
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Governmental Agency Mortgage-Backed Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 791,949
|
1,309,321
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | U.S. Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 185,333
|
375,353
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Foreign Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 63,043
|
114,047
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Investment Grade | A- Rating or Higher |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 990,592
|
|
| Percentage of investments in unrealized loss position |
82.30%
|
|
| Debt Securities | Investment Grade | A- Rating or Higher | U.S. Treasury Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 37,870
|
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Investment Grade | A- Rating or Higher | Municipal Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 64,560
|
|
| Percentage of investments in unrealized loss position |
86.60%
|
|
| Debt Securities | Investment Grade | A- Rating or Higher | Foreign Government Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 7,567
|
|
| Percentage of investments in unrealized loss position |
53.10%
|
|
| Debt Securities | Investment Grade | A- Rating or Higher | Governmental Agency Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 37,002
|
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Investment Grade | A- Rating or Higher | Governmental Agency Mortgage-Backed Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 791,949
|
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Debt Securities | Investment Grade | A- Rating or Higher | U.S. Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 37,102
|
|
| Percentage of investments in unrealized loss position |
20.10%
|
|
| Debt Securities | Investment Grade | A- Rating or Higher | Foreign Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 14,542
|
|
| Percentage of investments in unrealized loss position |
23.10%
|
|
| Debt Securities | Investment Grade | BBB+ to BBB- Ratings |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 92,581
|
|
| Percentage of investments in unrealized loss position |
7.70%
|
|
| Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Municipal Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 5,034
|
|
| Percentage of investments in unrealized loss position |
6.70%
|
|
| Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Foreign Government Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 5,566
|
|
| Percentage of investments in unrealized loss position |
39.10%
|
|
| Debt Securities | Investment Grade | BBB+ to BBB- Ratings | U.S. Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 59,762
|
|
| Percentage of investments in unrealized loss position |
32.20%
|
|
| Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Foreign Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 22,219
|
|
| Percentage of investments in unrealized loss position |
35.20%
|
|
| Debt Securities | Non-Investment Grade |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 120,883
|
|
| Percentage of investments in unrealized loss position |
10.00%
|
|
| Debt Securities | Non-Investment Grade | Municipal Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 5,020
|
|
| Percentage of investments in unrealized loss position |
6.70%
|
|
| Debt Securities | Non-Investment Grade | Foreign Government Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 1,112
|
|
| Percentage of investments in unrealized loss position |
7.80%
|
|
| Debt Securities | Non-Investment Grade | U.S. Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 88,469
|
|
| Percentage of investments in unrealized loss position |
47.70%
|
|
| Debt Securities | Non-Investment Grade | Foreign Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 26,282
|
|
| Percentage of investments in unrealized loss position |
41.70%
|
|
| Equity securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 155,441
|
$ 223,222
|
| Equity securities | Preferred stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 15,027
|
|
| Percentage of investments in unrealized loss position |
100.00%
|
|
| Equity securities | Investment Grade | BBB+ to BBB- Ratings | Preferred stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 8,869
|
|
| Percentage of investments in unrealized loss position |
59.00%
|
|
| Equity securities | Non-Investment Grade | Preferred stocks |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Estimated fair value, Unrealized loss position |
$ 6,158
|
|
| Percentage of investments in unrealized loss position |
41.00%
|
|