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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 9,000,753		$ 6,121,004
Debt Securities, Allowance for credit losses	(101)	$ (105)	(132)	$ (821)	$ (1,380)	$ 0
Debt Securities, Gross unrealized gains	148,393		237,736
Debt Securities, Gross unrealized losses	(53,239)		(3,786)
Debt securities, Estimated fair value	9,095,806		6,354,822
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	172,255		80,172
Debt Securities, Allowance for credit losses	0		0
Debt Securities, Gross unrealized gains	695		778
Debt Securities, Gross unrealized losses	(1,207)		(104)
Debt securities, Estimated fair value	171,743		80,846
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,564,102		1,168,425
Debt Securities, Allowance for credit losses	0		0
Debt Securities, Gross unrealized gains	60,705		80,953
Debt Securities, Gross unrealized losses	(11,006)		(570)
Debt securities, Estimated fair value	1,613,801		1,248,808
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	232,495		194,042
Debt Securities, Allowance for credit losses	0		0
Debt Securities, Gross unrealized gains	2,209		6,004
Debt Securities, Gross unrealized losses	(3,329)		(516)
Debt securities, Estimated fair value	231,375		199,530
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	208,094		254,248
Debt Securities, Allowance for credit losses	0		0
Debt Securities, Gross unrealized gains	3,975		9,869
Debt Securities, Gross unrealized losses	(562)		(195)
Debt securities, Estimated fair value	211,507		263,922
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	5,360,175		3,401,737
Debt Securities, Allowance for credit losses	0		0
Debt Securities, Gross unrealized gains	44,912		74,549
Debt Securities, Gross unrealized losses	(31,929)		(1,668)
Debt securities, Estimated fair value	5,373,158		3,474,618
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,002,449		637,808
Debt Securities, Allowance for credit losses	(101)		(119)
Debt Securities, Gross unrealized gains	23,760		43,505
Debt Securities, Gross unrealized losses	(3,719)		(497)
Debt securities, Estimated fair value	1,022,389		680,697
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	461,183		384,572
Debt Securities, Allowance for credit losses	0		(13)
Debt Securities, Gross unrealized gains	12,137		22,078
Debt Securities, Gross unrealized losses	(1,487)		(236)
Debt securities, Estimated fair value	$ 471,833		$ 406,401