Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 2,893,627	$ 1,437,759
Unrealized losses, Less than 12 months	(47,352)	(10,863)
Estimated fair value, 12 months or longer	1,518,869	1,206,188
Unrealized losses, 12 months or longer	(57,227)	(21,830)
Estimated fair value, Total	4,412,496	2,643,947
Unrealized losses, Total	(104,579)	(32,693)
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	138,105	78,605
Unrealized losses, Less than 12 months	(2,976)	(511)
Estimated fair value, 12 months or longer	49,131	37,498
Unrealized losses, 12 months or longer	(2,057)	(739)
Estimated fair value, Total	187,236	116,103
Unrealized losses, Total	(5,033)	(1,250)
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	508,857	279,292
Unrealized losses, Less than 12 months	(7,747)	(1,714)
Estimated fair value, 12 months or longer	296,302	226,895
Unrealized losses, 12 months or longer	(15,226)	(5,680)
Estimated fair value, Total	805,159	506,187
Unrealized losses, Total	(22,973)	(7,394)
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	101,286	98,942
Unrealized losses, Less than 12 months	(1,250)	(972)
Estimated fair value, 12 months or longer	33,004	6,678
Unrealized losses, 12 months or longer	(1,268)	(249)
Estimated fair value, Total	134,290	105,620
Unrealized losses, Total	(2,518)	(1,221)
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	183,040	55,707
Unrealized losses, Less than 12 months	(3,260)	(409)
Estimated fair value, 12 months or longer	136,979	93,737
Unrealized losses, 12 months or longer	(5,165)	(1,913)
Estimated fair value, Total	320,019	149,444
Unrealized losses, Total	(8,425)	(2,322)
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,367,730	671,871
Unrealized losses, Less than 12 months	(21,748)	(4,868)
Estimated fair value, 12 months or longer	877,474	774,959
Unrealized losses, 12 months or longer	(28,015)	(11,720)
Estimated fair value, Total	2,245,204	1,446,830
Unrealized losses, Total	(49,763)	(16,588)
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	442,049	171,817
Unrealized losses, Less than 12 months	(7,825)	(1,568)
Estimated fair value, 12 months or longer	86,252	60,724
Unrealized losses, 12 months or longer	(4,322)	(1,394)
Estimated fair value, Total	528,301	232,541
Unrealized losses, Total	(12,147)	(2,962)
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	152,560	81,525
Unrealized losses, Less than 12 months	(2,546)	(821)
Estimated fair value, 12 months or longer	39,727	5,697
Unrealized losses, 12 months or longer	(1,174)	(135)
Estimated fair value, Total	192,287	87,222
Unrealized losses, Total	$ (3,720)	$ (956)