Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 638,150	$ 1,672,754
Unrealized losses, Less than 12 months	(3,141)	(25,709)
Estimated fair value, 12 months or longer	1,103,091	1,700,760
Unrealized losses, 12 months or longer	(9,551)	(44,991)
Estimated fair value, Total	1,741,241	3,373,514
Unrealized losses, Total	(12,692)	(70,700)
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	8,921	19,749
Unrealized losses, Less than 12 months	(11)	(85)
Estimated fair value, 12 months or longer	14,863	55,615
Unrealized losses, 12 months or longer	(38)	(1,054)
Estimated fair value, Total	23,784	75,364
Unrealized losses, Total	(49)	(1,139)
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	9,029	172,387
Unrealized losses, Less than 12 months	(46)	(1,772)
Estimated fair value, 12 months or longer	59,364	369,139
Unrealized losses, 12 months or longer	(159)	(10,537)
Estimated fair value, Total	68,393	541,526
Unrealized losses, Total	(205)	(12,309)
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	47,048	23,654
Unrealized losses, Less than 12 months	(79)	(1,037)
Estimated fair value, 12 months or longer	13,020	42,119
Unrealized losses, 12 months or longer	(71)	(1,122)
Estimated fair value, Total	60,068	65,773
Unrealized losses, Total	(150)	(2,159)
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	15,282	56,270
Unrealized losses, Less than 12 months	(67)	(748)
Estimated fair value, 12 months or longer	45,817	90,631
Unrealized losses, 12 months or longer	(447)	(3,345)
Estimated fair value, Total	61,099	146,901
Unrealized losses, Total	(514)	(4,093)
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	466,134	850,459
Unrealized losses, Less than 12 months	(1,963)	(6,955)
Estimated fair value, 12 months or longer	891,182	982,610
Unrealized losses, 12 months or longer	(7,977)	(22,061)
Estimated fair value, Total	1,357,316	1,833,069
Unrealized losses, Total	(9,940)	(29,016)
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	66,367	374,473
Unrealized losses, Less than 12 months	(575)	(10,537)
Estimated fair value, 12 months or longer	48,940	109,844
Unrealized losses, 12 months or longer	(476)	(4,962)
Estimated fair value, Total	115,307	484,317
Unrealized losses, Total	(1,051)	(15,499)
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	25,369	175,762
Unrealized losses, Less than 12 months	(400)	(4,575)
Estimated fair value, 12 months or longer	29,905	50,802
Unrealized losses, 12 months or longer	(383)	(1,910)
Estimated fair value, Total	55,274	226,564
Unrealized losses, Total	$ (783)	$ (6,485)