Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt and Equity Securities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 2,020,830	$ 2,596,677
Unrealized losses, Less than 12 months	(33,249)	(63,300)
Estimated fair value, 12 months or longer	716,723	540,830
Unrealized losses, 12 months or longer	(11,433)	(16,204)
Estimated fair value, Total	2,737,553	3,137,507
Unrealized losses, Total	(44,682)	(79,504)
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,949,926	2,526,416
Unrealized losses, Less than 12 months	(31,798)	(62,127)
Estimated fair value, 12 months or longer	693,464	481,811
Unrealized losses, 12 months or longer	(9,615)	(9,333)
Estimated fair value, Total	2,643,390	3,008,227
Unrealized losses, Total	(41,413)	(71,460)
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	103,246	111,748
Unrealized losses, Less than 12 months	(1,455)	(4,466)
Estimated fair value, 12 months or longer	186	0
Unrealized losses, 12 months or longer	(29)	0
Estimated fair value, Total	103,432	111,748
Unrealized losses, Total	(1,484)	(4,466)
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	417,676	635,531
Unrealized losses, Less than 12 months	(12,676)	(26,317)
Estimated fair value, 12 months or longer	21,707	16,485
Unrealized losses, 12 months or longer	(809)	(349)
Estimated fair value, Total	439,383	652,016
Unrealized losses, Total	(13,485)	(26,666)
Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	94,332	63,044
Unrealized losses, Less than 12 months	(1,149)	(2,371)
Estimated fair value, 12 months or longer	350	324
Unrealized losses, 12 months or longer	(42)	(68)
Estimated fair value, Total	94,682	63,368
Unrealized losses, Total	(1,191)	(2,439)
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	154,574	148,112
Unrealized losses, Less than 12 months	(2,337)	(4,166)
Estimated fair value, 12 months or longer	5,353	0
Unrealized losses, 12 months or longer	(479)	0
Estimated fair value, Total	159,927	148,112
Unrealized losses, Total	(2,816)	(4,166)
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	977,367	1,295,790
Unrealized losses, Less than 12 months	(11,731)	(19,097)
Estimated fair value, 12 months or longer	649,022	432,349
Unrealized losses, 12 months or longer	(7,646)	(7,695)
Estimated fair value, Total	1,626,389	1,728,139
Unrealized losses, Total	(19,377)	(26,792)
Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	160,663	193,533
Unrealized losses, Less than 12 months	(2,179)	(4,560)
Estimated fair value, 12 months or longer	15,406	24,499
Unrealized losses, 12 months or longer	(552)	(881)
Estimated fair value, Total	176,069	218,032
Unrealized losses, Total	(2,731)	(5,441)
Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	42,068	78,658
Unrealized losses, Less than 12 months	(271)	(1,150)
Estimated fair value, 12 months or longer	1,440	8,154
Unrealized losses, 12 months or longer	(58)	(340)
Estimated fair value, Total	43,508	86,812
Unrealized losses, Total	(329)	(1,490)
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	70,904	70,261
Unrealized losses, Less than 12 months	(1,451)	(1,173)
Estimated fair value, 12 months or longer	23,259	59,019
Unrealized losses, 12 months or longer	(1,818)	(6,871)
Estimated fair value, Total	94,163	129,280
Unrealized losses, Total	$ (3,269)	$ (8,044)