Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 4,478,534	$ 513,733
Unrealized losses, Less than 12 months	(49,992)	(3,190)
Estimated fair value, 12 months or longer	53,668	102,106
Unrealized losses, 12 months or longer	(3,247)	(596)
Estimated fair value, Total	4,532,202	615,839
Unrealized losses, Total	(53,239)	(3,786)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	52,927	7,744
Unrealized losses, Less than 12 months	(916)	(104)
Estimated fair value, 12 months or longer	6,518	0
Unrealized losses, 12 months or longer	(291)	0
Estimated fair value, Total	59,445	7,744
Unrealized losses, Total	(1,207)	(104)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	570,038	74,045
Unrealized losses, Less than 12 months	(10,724)	(570)
Estimated fair value, 12 months or longer	6,711	0
Unrealized losses, 12 months or longer	(282)	0
Estimated fair value, Total	576,749	74,045
Unrealized losses, Total	(11,006)	(570)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	125,918	67,094
Unrealized losses, Less than 12 months	(1,094)	(516)
Estimated fair value, 12 months or longer	29,032	0
Unrealized losses, 12 months or longer	(2,235)	0
Estimated fair value, Total	154,950	67,094
Unrealized losses, Total	(3,329)	(516)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	72,325	15,353
Unrealized losses, Less than 12 months	(469)	(195)
Estimated fair value, 12 months or longer	3,222	0
Unrealized losses, 12 months or longer	(93)	0
Estimated fair value, Total	75,547	15,353
Unrealized losses, Total	(562)	(195)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	3,070,056	287,947
Unrealized losses, Less than 12 months	(31,928)	(1,089)
Estimated fair value, 12 months or longer	103	100,473
Unrealized losses, 12 months or longer	(1)	(579)
Estimated fair value, Total	3,070,159	388,420
Unrealized losses, Total	(31,929)	(1,668)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	425,854	42,508
Unrealized losses, Less than 12 months	(3,388)	(484)
Estimated fair value, 12 months or longer	6,848	1,357
Unrealized losses, 12 months or longer	(331)	(13)
Estimated fair value, Total	432,702	43,865
Unrealized losses, Total	(3,719)	(497)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	161,416	19,042
Unrealized losses, Less than 12 months	(1,473)	(232)
Estimated fair value, 12 months or longer	1,234	276
Unrealized losses, 12 months or longer	(14)	(4)
Estimated fair value, Total	162,650	19,318
Unrealized losses, Total	$ (1,487)	$ (236)