Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt and Equity Securities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 2,149,742	$ 2,596,677
Unrealized losses, Less than 12 months	(48,047)	(63,300)
Estimated fair value, 12 months or longer	539,683	540,830
Unrealized losses, 12 months or longer	(10,391)	(16,204)
Estimated fair value, Total	2,689,425	3,137,507
Unrealized losses, Total	(58,438)	(79,504)
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	2,119,014	2,526,416
Unrealized losses, Less than 12 months	(47,308)	(62,127)
Estimated fair value, 12 months or longer	478,600	481,811
Unrealized losses, 12 months or longer	(7,849)	(9,333)
Estimated fair value, Total	2,597,614	3,008,227
Unrealized losses, Total	(55,157)	(71,460)
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	101,603	111,748
Unrealized losses, Less than 12 months	(1,999)	(4,466)
Estimated fair value, 12 months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Estimated fair value, Total	101,603	111,748
Unrealized losses, Total	(1,999)	(4,466)
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	502,408	635,531
Unrealized losses, Less than 12 months	(20,827)	(26,317)
Estimated fair value, 12 months or longer	19,279	16,485
Unrealized losses, 12 months or longer	(241)	(349)
Estimated fair value, Total	521,687	652,016
Unrealized losses, Total	(21,068)	(26,666)
Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	35,326	63,044
Unrealized losses, Less than 12 months	(995)	(2,371)
Estimated fair value, 12 months or longer	356	324
Unrealized losses, 12 months or longer	(36)	(68)
Estimated fair value, Total	35,682	63,368
Unrealized losses, Total	(1,031)	(2,439)
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	148,314	148,112
Unrealized losses, Less than 12 months	(3,852)	(4,166)
Estimated fair value, 12 months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Estimated fair value, Total	148,314	148,112
Unrealized losses, Total	(3,852)	(4,166)
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,116,770	1,295,790
Unrealized losses, Less than 12 months	(15,302)	(19,097)
Estimated fair value, 12 months or longer	437,981	432,349
Unrealized losses, 12 months or longer	(6,726)	(7,695)
Estimated fair value, Total	1,554,751	1,728,139
Unrealized losses, Total	(22,028)	(26,792)
Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	166,418	193,533
Unrealized losses, Less than 12 months	(3,810)	(4,560)
Estimated fair value, 12 months or longer	18,903	24,499
Unrealized losses, 12 months or longer	(704)	(881)
Estimated fair value, Total	185,321	218,032
Unrealized losses, Total	(4,514)	(5,441)
Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	48,175	78,658
Unrealized losses, Less than 12 months	(523)	(1,150)
Estimated fair value, 12 months or longer	2,081	8,154
Unrealized losses, 12 months or longer	(142)	(340)
Estimated fair value, Total	50,256	86,812
Unrealized losses, Total	(665)	(1,490)
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	30,728	70,261
Unrealized losses, Less than 12 months	(739)	(1,173)
Estimated fair value, 12 months or longer	61,083	59,019
Unrealized losses, 12 months or longer	(2,542)	(6,871)
Estimated fair value, Total	91,811	129,280
Unrealized losses, Total	$ (3,281)	$ (8,044)