Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 795,192	$ 1,672,754
Unrealized losses, Less than 12 months	(5,208)	(25,709)
Estimated fair value, 12 months or longer	761,354	1,700,760
Unrealized losses, 12 months or longer	(6,898)	(44,991)
Estimated fair value, Total	1,556,546	3,373,514
Unrealized losses, Total	(12,106)	(70,700)
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	42,776	19,749
Unrealized losses, Less than 12 months	(271)	(85)
Estimated fair value, 12 months or longer	4,925	55,615
Unrealized losses, 12 months or longer	(12)	(1,054)
Estimated fair value, Total	47,701	75,364
Unrealized losses, Total	(283)	(1,139)
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	42,084	172,387
Unrealized losses, Less than 12 months	(111)	(1,772)
Estimated fair value, 12 months or longer	3,397	369,139
Unrealized losses, 12 months or longer	(18)	(10,537)
Estimated fair value, Total	45,481	541,526
Unrealized losses, Total	(129)	(12,309)
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	26,049	23,654
Unrealized losses, Less than 12 months	(78)	(1,037)
Estimated fair value, 12 months or longer	10,801	42,119
Unrealized losses, 12 months or longer	(38)	(1,122)
Estimated fair value, Total	36,850	65,773
Unrealized losses, Total	(116)	(2,159)
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	14,935	56,270
Unrealized losses, Less than 12 months	(32)	(748)
Estimated fair value, 12 months or longer	32,537	90,631
Unrealized losses, 12 months or longer	(83)	(3,345)
Estimated fair value, Total	47,472	146,901
Unrealized losses, Total	(115)	(4,093)
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	581,727	850,459
Unrealized losses, Less than 12 months	(3,900)	(6,955)
Estimated fair value, 12 months or longer	683,684	982,610
Unrealized losses, 12 months or longer	(6,000)	(22,061)
Estimated fair value, Total	1,265,411	1,833,069
Unrealized losses, Total	(9,900)	(29,016)
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	69,127	374,473
Unrealized losses, Less than 12 months	(668)	(10,537)
Estimated fair value, 12 months or longer	13,933	109,844
Unrealized losses, 12 months or longer	(511)	(4,962)
Estimated fair value, Total	83,060	484,317
Unrealized losses, Total	(1,179)	(15,499)
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	18,494	175,762
Unrealized losses, Less than 12 months	(148)	(4,575)
Estimated fair value, 12 months or longer	12,077	50,802
Unrealized losses, 12 months or longer	(236)	(1,910)
Estimated fair value, Total	30,571	226,564
Unrealized losses, Total	$ (384)	$ (6,485)