Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt and Equity Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,147,431	$ 2,195,994
Unrealized losses, Less than 12 months	(6,383)	(40,894)
Estimated fair value, 12 months or longer	474,107	303,099
Unrealized losses, 12 months or longer	(13,002)	(14,340)
Estimated fair value, Total	1,621,538	2,499,093
Unrealized losses, Total	(19,385)	(55,234)
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,139,715	2,004,746
Unrealized losses, Less than 12 months	(6,309)	(28,826)
Estimated fair value, 12 months or longer	397,390	271,125
Unrealized losses, 12 months or longer	(5,103)	(8,436)
Estimated fair value, Total	1,537,105	2,275,871
Unrealized losses, Total	(11,412)	(37,262)
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	55,705	105,701
Unrealized losses, Less than 12 months	(463)	(1,285)
Estimated fair value, 12 months or longer	0	1,654
Unrealized losses, 12 months or longer	0	(16)
Estimated fair value, Total	55,705	107,355
Unrealized losses, Total	(463)	(1,301)
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	223,714	133,465
Unrealized losses, Less than 12 months	(1,941)	(733)
Estimated fair value, 12 months or longer	16,401	13,190
Unrealized losses, 12 months or longer	(282)	(112)
Estimated fair value, Total	240,115	146,655
Unrealized losses, Total	(2,223)	(845)
Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	32,714	13,601
Unrealized losses, Less than 12 months	(313)	(890)
Estimated fair value, 12 months or longer	363	267
Unrealized losses, 12 months or longer	(29)	(125)
Estimated fair value, Total	33,077	13,868
Unrealized losses, Total	(342)	(1,015)
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	749,900	1,096,301
Unrealized losses, Less than 12 months	(2,654)	(9,424)
Estimated fair value, 12 months or longer	339,896	213,020
Unrealized losses, 12 months or longer	(3,509)	(5,615)
Estimated fair value, Total	1,089,796	1,309,321
Unrealized losses, Total	(6,163)	(15,039)
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	5,706	191,035
Unrealized losses, Less than 12 months	(123)	(2,497)
Estimated fair value, 12 months or longer	0	18,237
Unrealized losses, 12 months or longer	0	(304)
Estimated fair value, Total	5,706	209,272
Unrealized losses, Total	(123)	(2,801)
Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	50,855	361,842
Unrealized losses, Less than 12 months	(634)	(11,272)
Estimated fair value, 12 months or longer	30,740	13,511
Unrealized losses, 12 months or longer	(1,001)	(1,211)
Estimated fair value, Total	81,595	375,353
Unrealized losses, Total	(1,635)	(12,483)
Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	21,121	102,801
Unrealized losses, Less than 12 months	(181)	(2,725)
Estimated fair value, 12 months or longer	9,990	11,246
Unrealized losses, 12 months or longer	(282)	(1,053)
Estimated fair value, Total	31,111	114,047
Unrealized losses, Total	(463)	(3,778)
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	7,716	191,248
Unrealized losses, Less than 12 months	(74)	(12,068)
Estimated fair value, 12 months or longer	76,717	31,974
Unrealized losses, 12 months or longer	(7,899)	(5,904)
Estimated fair value, Total	84,433	223,222
Unrealized losses, Total	$ (7,973)	$ (17,972)