Debt and Equity Securities (Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized Cost	$ 4,541,515	$ 4,290,864
Debt securities, Estimated fair value	4,592,170	4,279,347
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	35,270
Amortized cost, Due after one through five years	68,799
Amortized cost, Due after five through ten years	16,147
Amortized cost, Due after ten years	28,592
Debt Securities, Amortized Cost	148,808	130,252
Estimated fair value, Due in one year or less	35,293
Estimated fair value, Due after one through five years	69,127
Estimated fair value, Due after five through ten years	16,660
Estimated fair value, Due after ten years	29,354
Debt securities, Estimated fair value	150,434	129,372
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	47,544
Amortized cost, Due after one through five years	331,550
Amortized cost, Due after five through ten years	214,529
Amortized cost, Due after ten years	313,272
Debt Securities, Amortized Cost	906,895	692,000
Estimated fair value, Due in one year or less	47,615
Estimated fair value, Due after one through five years	337,258
Estimated fair value, Due after five through ten years	221,613
Estimated fair value, Due after ten years	317,751
Debt securities, Estimated fair value	924,237	703,795
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	4,447
Amortized cost, Due after one through five years	107,304
Amortized cost, Due after five through ten years	11,240
Amortized cost, Due after ten years	20,862
Debt Securities, Amortized Cost	143,853	129,984
Estimated fair value, Due in one year or less	4,482
Estimated fair value, Due after one through five years	108,323
Estimated fair value, Due after five through ten years	11,702
Estimated fair value, Due after ten years	20,851
Debt securities, Estimated fair value	145,358	130,101
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	20,771
Amortized cost, Due after one through five years	82,989
Amortized cost, Due after five through ten years	47,203
Amortized cost, Due after ten years	41,467
Debt Securities, Amortized Cost	192,430	419,869
Estimated fair value, Due in one year or less	20,781
Estimated fair value, Due after one through five years	83,238
Estimated fair value, Due after five through ten years	47,561
Estimated fair value, Due after ten years	43,597
Debt securities, Estimated fair value	195,177	418,091
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	15,513
Amortized cost, Due after one through five years	318,116
Amortized cost, Due after five through ten years	274,598
Amortized cost, Due after ten years	66,424
Debt Securities, Amortized Cost	674,651	642,869
Estimated fair value, Due in one year or less	15,578
Estimated fair value, Due after one through five years	324,386
Estimated fair value, Due after five through ten years	281,216
Estimated fair value, Due after ten years	69,558
Debt securities, Estimated fair value	690,738	634,683
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	9,521
Amortized cost, Due after one through five years	114,704
Amortized cost, Due after five through ten years	110,180
Amortized cost, Due after ten years	10,822
Debt Securities, Amortized Cost	245,227	210,162
Estimated fair value, Due in one year or less	9,532
Estimated fair value, Due after one through five years	116,436
Estimated fair value, Due after five through ten years	112,776
Estimated fair value, Due after ten years	11,685
Debt securities, Estimated fair value	250,429	$ 207,632
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	133,066
Amortized cost, Due after one through five years	1,023,462
Amortized cost, Due after five through ten years	673,897
Amortized cost, Due after ten years	481,439
Debt Securities, Amortized Cost	2,311,864
Estimated fair value, Due in one year or less	133,281
Estimated fair value, Due after one through five years	1,038,768
Estimated fair value, Due after five through ten years	691,528
Estimated fair value, Due after ten years	492,796
Debt securities, Estimated fair value	2,356,373
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	2,229,651
Estimated fair value	$ 2,235,797