Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 2,546,049	$ 1,437,759
Unrealized losses, Less than 12 months	(44,907)	(10,863)
Estimated fair value, 12 months or longer	1,252,434	1,206,188
Unrealized losses, 12 months or longer	(43,129)	(21,830)
Estimated fair value, Total	3,798,483	2,643,947
Unrealized losses, Total	(88,036)	(32,693)
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	111,297	78,605
Unrealized losses, Less than 12 months	(1,927)	(511)
Estimated fair value, 12 months or longer	34,287	37,498
Unrealized losses, 12 months or longer	(1,131)	(739)
Estimated fair value, Total	145,584	116,103
Unrealized losses, Total	(3,058)	(1,250)
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	473,525	279,292
Unrealized losses, Less than 12 months	(5,656)	(1,714)
Estimated fair value, 12 months or longer	227,275	226,895
Unrealized losses, 12 months or longer	(12,358)	(5,680)
Estimated fair value, Total	700,800	506,187
Unrealized losses, Total	(18,014)	(7,394)
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	72,240	98,942
Unrealized losses, Less than 12 months	(1,265)	(972)
Estimated fair value, 12 months or longer	35,537	6,678
Unrealized losses, 12 months or longer	(1,409)	(249)
Estimated fair value, Total	107,777	105,620
Unrealized losses, Total	(2,674)	(1,221)
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	118,367	55,707
Unrealized losses, Less than 12 months	(2,185)	(409)
Estimated fair value, 12 months or longer	116,299	93,737
Unrealized losses, 12 months or longer	(3,613)	(1,913)
Estimated fair value, Total	234,666	149,444
Unrealized losses, Total	(5,798)	(2,322)
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,079,083	671,871
Unrealized losses, Less than 12 months	(19,034)	(4,868)
Estimated fair value, 12 months or longer	757,050	774,959
Unrealized losses, 12 months or longer	(20,415)	(11,720)
Estimated fair value, Total	1,836,133	1,446,830
Unrealized losses, Total	(39,449)	(16,588)
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	511,761	171,817
Unrealized losses, Less than 12 months	(11,007)	(1,568)
Estimated fair value, 12 months or longer	60,723	60,724
Unrealized losses, 12 months or longer	(3,630)	(1,394)
Estimated fair value, Total	572,484	232,541
Unrealized losses, Total	(14,637)	(2,962)
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	179,776	81,525
Unrealized losses, Less than 12 months	(3,833)	(821)
Estimated fair value, 12 months or longer	21,263	5,697
Unrealized losses, 12 months or longer	(573)	(135)
Estimated fair value, Total	201,039	87,222
Unrealized losses, Total	$ (4,406)	$ (956)