Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,055,306	$ 1,672,754
Unrealized losses, Less than 12 months	(7,332)	(25,709)
Estimated fair value, 12 months or longer	1,780,457	1,700,760
Unrealized losses, 12 months or longer	(24,936)	(44,991)
Estimated fair value, Total	2,835,763	3,373,514
Unrealized losses, Total	(32,268)	(70,700)
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	2,243	19,749
Unrealized losses, Less than 12 months	(5)	(85)
Estimated fair value, 12 months or longer	39,855	55,615
Unrealized losses, 12 months or longer	(319)	(1,054)
Estimated fair value, Total	42,098	75,364
Unrealized losses, Total	(324)	(1,139)
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	8,130	172,387
Unrealized losses, Less than 12 months	(10)	(1,772)
Estimated fair value, 12 months or longer	258,588	369,139
Unrealized losses, 12 months or longer	(2,187)	(10,537)
Estimated fair value, Total	266,718	541,526
Unrealized losses, Total	(2,197)	(12,309)
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	18,842	23,654
Unrealized losses, Less than 12 months	(85)	(1,037)
Estimated fair value, 12 months or longer	30,536	42,119
Unrealized losses, 12 months or longer	(387)	(1,122)
Estimated fair value, Total	49,378	65,773
Unrealized losses, Total	(472)	(2,159)
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	37,180	56,270
Unrealized losses, Less than 12 months	(188)	(748)
Estimated fair value, 12 months or longer	107,168	90,631
Unrealized losses, 12 months or longer	(2,184)	(3,345)
Estimated fair value, Total	144,348	146,901
Unrealized losses, Total	(2,372)	(4,093)
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	789,483	850,459
Unrealized losses, Less than 12 months	(4,251)	(6,955)
Estimated fair value, 12 months or longer	1,115,558	982,610
Unrealized losses, 12 months or longer	(16,559)	(22,061)
Estimated fair value, Total	1,905,041	1,833,069
Unrealized losses, Total	(20,810)	(29,016)
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	147,143	374,473
Unrealized losses, Less than 12 months	(2,324)	(10,537)
Estimated fair value, 12 months or longer	149,021	109,844
Unrealized losses, 12 months or longer	(2,132)	(4,962)
Estimated fair value, Total	296,164	484,317
Unrealized losses, Total	(4,456)	(15,499)
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	52,285	175,762
Unrealized losses, Less than 12 months	(469)	(4,575)
Estimated fair value, 12 months or longer	79,731	50,802
Unrealized losses, 12 months or longer	(1,168)	(1,910)
Estimated fair value, Total	132,016	226,564
Unrealized losses, Total	$ (1,637)	$ (6,485)