Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 15,331,053		$ 14,898,522
Debt securities, gross unrealized gains		583,300		762,200
Debt securities, gross unrealized losses		(49,100)		(39,600)
Debt securities, allowance for credit losses		(361)	$ (600)	(2,579)	$ (8,900)	$ (31,400)	$ 0
Debt securities, fair value		15,864,867		15,618,470
Amortized Cost or Cost		15,710,100		15,612,700
Gross Unrealized Gains		583,300		762,200
Gross Unrealized Losses		(49,100)		(39,600)
Allowance for Credit Losses		(400)		(2,600)
Fair Value		16,243,900		16,332,700
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,024,300		1,317,600
Debt securities, gross unrealized gains		31,000		44,000
Debt securities, gross unrealized losses		(8,400)		(1,500)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		2,046,900		1,360,100
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,452,100		2,489,100
Debt securities, gross unrealized gains		150,100		168,300
Debt securities, gross unrealized losses		(1,400)		(600)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		2,600,800		2,656,800
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		817,800		846,600
Debt securities, gross unrealized gains		20,000		33,400
Debt securities, gross unrealized losses		(4,600)		(500)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		833,200		879,500
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,128,500		3,262,000
Debt securities, gross unrealized gains		215,300		289,700
Debt securities, gross unrealized losses		(10,800)		(3,500)
Debt securities, allowance for credit losses		(400)		(2,500)
Debt securities, fair value		3,332,600		3,545,700
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,260,000		1,268,300
Debt securities, gross unrealized gains		35,200		55,600
Debt securities, gross unrealized losses		(4,100)		(700)
Debt securities, allowance for credit losses		0		(100)
Debt securities, fair value		1,291,100		1,323,100
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,906,300		2,533,600
Debt securities, gross unrealized gains		52,100		78,300
Debt securities, gross unrealized losses		(10,900)		(500)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value	[1]	1,947,500		2,611,400
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		869,400		852,600
Debt securities, gross unrealized gains		41,200		45,300
Debt securities, gross unrealized losses		(2,100)		(7,600)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		908,500		890,300
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	2,872,700		2,328,700
Debt securities, gross unrealized gains	[2]	38,400		47,600
Debt securities, gross unrealized losses	[2]	(6,800)		(24,700)
Debt securities, allowance for credit losses	[2]	0		0
Debt securities, fair value	[2]	2,904,300		2,351,600
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		379,000		714,200
Gross Unrealized Gains		0		0
Gross Unrealized Losses		0		0
Allowance for Credit Losses		0		0
Fair Value		$ 379,000		$ 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 $1,356.3 million and $878.4 million of collateralized loan obligations as of June 30, 2021 and December 31, 2020, respectively.