Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 2,911,884	$ 513,733
Unrealized losses, Less than 12 months	(68,642)	(3,190)
Estimated fair value, 12 months or longer	12,670	102,106
Unrealized losses, 12 months or longer	(335)	(596)
Estimated fair value, Total	2,924,554	615,839
Unrealized losses, Total	(68,977)	(3,786)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	109,133	7,744
Unrealized losses, Less than 12 months	(2,750)	(104)
Estimated fair value, 12 months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Estimated fair value, Total	109,133	7,744
Unrealized losses, Total	(2,750)	(104)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	466,304	74,045
Unrealized losses, Less than 12 months	(17,569)	(570)
Estimated fair value, 12 months or longer	2,391	0
Unrealized losses, 12 months or longer	(109)	0
Estimated fair value, Total	468,695	74,045
Unrealized losses, Total	(17,678)	(570)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	91,324	67,094
Unrealized losses, Less than 12 months	(3,601)	(516)
Estimated fair value, 12 months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Estimated fair value, Total	91,324	67,094
Unrealized losses, Total	(3,601)	(516)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	69,387	15,353
Unrealized losses, Less than 12 months	(2,031)	(195)
Estimated fair value, 12 months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Estimated fair value, Total	69,387	15,353
Unrealized losses, Total	(2,031)	(195)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,896,533	287,947
Unrealized losses, Less than 12 months	(33,607)	(1,089)
Estimated fair value, 12 months or longer	709	100,473
Unrealized losses, 12 months or longer	(2)	(579)
Estimated fair value, Total	1,897,242	388,420
Unrealized losses, Total	(33,609)	(1,668)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	165,797	42,508
Unrealized losses, Less than 12 months	(5,009)	(484)
Estimated fair value, 12 months or longer	6,198	1,357
Unrealized losses, 12 months or longer	(174)	(13)
Estimated fair value, Total	171,995	43,865
Unrealized losses, Total	(5,183)	(497)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	113,406	19,042
Unrealized losses, Less than 12 months	(4,075)	(232)
Estimated fair value, 12 months or longer	3,372	276
Unrealized losses, 12 months or longer	(50)	(4)
Estimated fair value, Total	116,778	19,318
Unrealized losses, Total	$ (4,125)	$ (236)