Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt and Equity Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,289,520	$ 2,596,677
Unrealized losses, Less than 12 months	(10,193)	(63,300)
Estimated fair value, 12 months or longer	1,272,712	540,830
Unrealized losses, 12 months or longer	(28,184)	(16,204)
Estimated fair value, Total	2,562,232	3,137,507
Unrealized losses, Total	(38,377)	(79,504)
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,253,113	2,526,416
Unrealized losses, Less than 12 months	(9,708)	(62,127)
Estimated fair value, 12 months or longer	1,244,597	481,811
Unrealized losses, 12 months or longer	(26,557)	(9,333)
Estimated fair value, Total	2,497,710	3,008,227
Unrealized losses, Total	(36,265)	(71,460)
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	61,500	111,748
Unrealized losses, Less than 12 months	(584)	(4,466)
Estimated fair value, 12 months or longer	20,177	0
Unrealized losses, 12 months or longer	(843)	0
Estimated fair value, Total	81,677	111,748
Unrealized losses, Total	(1,427)	(4,466)
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	124,976	635,531
Unrealized losses, Less than 12 months	(775)	(26,317)
Estimated fair value, 12 months or longer	247,001	16,485
Unrealized losses, 12 months or longer	(10,020)	(349)
Estimated fair value, Total	371,977	652,016
Unrealized losses, Total	(10,795)	(26,666)
Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	97,341	63,044
Unrealized losses, Less than 12 months	(1,058)	(2,371)
Estimated fair value, 12 months or longer	10,471	324
Unrealized losses, 12 months or longer	(399)	(68)
Estimated fair value, Total	107,812	63,368
Unrealized losses, Total	(1,457)	(2,439)
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	106,783	148,112
Unrealized losses, Less than 12 months	(778)	(4,166)
Estimated fair value, 12 months or longer	81,053	0
Unrealized losses, 12 months or longer	(2,113)	0
Estimated fair value, Total	187,836	148,112
Unrealized losses, Total	(2,891)	(4,166)
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	686,814	1,295,790
Unrealized losses, Less than 12 months	(4,572)	(19,097)
Estimated fair value, 12 months or longer	832,251	432,349
Unrealized losses, 12 months or longer	(11,828)	(7,695)
Estimated fair value, Total	1,519,065	1,728,139
Unrealized losses, Total	(16,400)	(26,792)
Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	113,028	193,533
Unrealized losses, Less than 12 months	(1,484)	(4,560)
Estimated fair value, 12 months or longer	50,152	24,499
Unrealized losses, 12 months or longer	(1,287)	(881)
Estimated fair value, Total	163,180	218,032
Unrealized losses, Total	(2,771)	(5,441)
Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	62,671	78,658
Unrealized losses, Less than 12 months	(457)	(1,150)
Estimated fair value, 12 months or longer	3,492	8,154
Unrealized losses, 12 months or longer	(67)	(340)
Estimated fair value, Total	66,163	86,812
Unrealized losses, Total	(524)	(1,490)
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	36,407	70,261
Unrealized losses, Less than 12 months	(485)	(1,173)
Estimated fair value, 12 months or longer	28,115	59,019
Unrealized losses, 12 months or longer	(1,627)	(6,871)
Estimated fair value, Total	64,522	129,280
Unrealized losses, Total	$ (2,112)	$ (8,044)