Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 14,514,691	$ 14,898,522
Debt securities, gross unrealized gains		528,300	762,200
Debt securities, gross unrealized losses		(96,900)	(39,600)
Debt securities, allowance for credit losses		(607)	(2,579)	$ (31,400)	$ 0
Debt securities, fair value		14,945,479	15,618,470
Amortized Cost or Cost		15,109,700	15,612,700
Gross Unrealized Gains		528,300	762,200
Gross Unrealized Losses		(96,900)	(39,600)
Allowance for Credit Losses		(600)	(2,600)
Fair Value		15,540,500	16,332,700
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,516,500	1,317,600
Debt securities, gross unrealized gains		25,000	44,000
Debt securities, gross unrealized losses		(15,900)	(1,500)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value		1,525,600	1,360,100
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,459,700	2,489,100
Debt securities, gross unrealized gains		128,900	168,300
Debt securities, gross unrealized losses		(9,600)	(600)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value		2,579,000	2,656,800
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		860,600	846,600
Debt securities, gross unrealized gains		20,800	33,400
Debt securities, gross unrealized losses		(6,900)	(500)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value		874,500	879,500
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,226,500	3,262,000
Debt securities, gross unrealized gains		188,700	289,700
Debt securities, gross unrealized losses		(23,200)	(3,500)
Debt securities, allowance for credit losses		(500)	(2,500)
Debt securities, fair value		3,391,500	3,545,700
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,280,200	1,268,300
Debt securities, gross unrealized gains		34,600	55,600
Debt securities, gross unrealized losses		(5,900)	(700)
Debt securities, allowance for credit losses		(100)	(100)
Debt securities, fair value		1,308,800	1,323,100
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,938,100	2,533,600
Debt securities, gross unrealized gains		58,400	78,300
Debt securities, gross unrealized losses		(19,400)	(500)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value	[1]	1,977,100	2,611,400
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		850,300	852,600
Debt securities, gross unrealized gains		33,200	45,300
Debt securities, gross unrealized losses		(6,800)	(7,600)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value		876,700	890,300
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	2,382,800	2,328,700
Debt securities, gross unrealized gains	[2]	38,700	47,600
Debt securities, gross unrealized losses	[2]	(9,200)	(24,700)
Debt securities, allowance for credit losses	[2]	0	0
Debt securities, fair value	[2]	2,412,300	2,351,600
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		595,000	714,200
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Allowance for Credit Losses		0	0
Fair Value		$ 595,000	$ 714,200

[1] Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.

[2] Includes $959.7 million and $878.4 million of collateralized loan obligations as of March 31, 2021 and December 31, 2020, respectively.