Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($)
$ in Thousands |
Mar. 31, 2016 |
Dec. 31, 2015 |
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
$ 4,655,582
|
$ 4,600,632
|
| Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
4,258,945
|
4,279,347
|
| Equity securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
396,637
|
321,285
|
| U.S. Treasury Bonds | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
144,394
|
129,372
|
| Municipal Bonds | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
699,524
|
703,795
|
| Foreign Government Bonds | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
129,577
|
130,101
|
| Governmental Agency Bonds | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
354,941
|
418,091
|
| Governmental Agency Mortgage-Backed Securities | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
2,044,958
|
2,055,673
|
| U.S. Corporate Debt Securities | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
659,865
|
634,683
|
| Foreign Corporate Debt Securities | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
225,686
|
207,632
|
| Preferred stocks | Equity securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
15,446
|
15,467
|
| Common Stock | Equity securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
381,191
|
305,818
|
| Level 1 |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
396,637
|
321,285
|
| Level 1 | Equity securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
396,637
|
321,285
|
| Level 1 | Preferred stocks | Equity securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
15,446
|
15,467
|
| Level 1 | Common Stock | Equity securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
381,191
|
305,818
|
| Level 2 |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
4,242,620
|
4,229,208
|
| Level 2 | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
4,242,620
|
4,229,208
|
| Level 2 | U.S. Treasury Bonds | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
144,394
|
129,372
|
| Level 2 | Municipal Bonds | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
699,524
|
703,795
|
| Level 2 | Foreign Government Bonds | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
129,577
|
130,101
|
| Level 2 | Governmental Agency Bonds | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
354,941
|
418,091
|
| Level 2 | Governmental Agency Mortgage-Backed Securities | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
2,044,958
|
2,055,673
|
| Level 2 | U.S. Corporate Debt Securities | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
644,872
|
591,116
|
| Level 2 | Foreign Corporate Debt Securities | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
224,354
|
201,060
|
| Level 3 |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
16,325
|
50,139
|
| Level 3 | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
16,325
|
50,139
|
| Level 3 | U.S. Corporate Debt Securities | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
14,993
|
43,567
|
| Level 3 | Foreign Corporate Debt Securities | Debt Securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |
|
|
| Available for sale securities, estimated fair value |
$ 1,332
|
$ 6,572
|