Amortized Cost and Fair Value of Available For Sale Securities (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		$ 15,441,754		$ 14,898,522
Debt securities, gross unrealized gains		509,500		762,200
Debt securities, gross unrealized losses		(66,000)		(39,600)
Debt securities, allowance for credit losses		(307)	$ (400)	(2,579)	$ (5,500)	$ (8,900)	$ 0
Debt securities, fair value		15,884,928		15,618,470
Amortized Cost or Cost		16,502,100		15,612,700
Gross Unrealized Gains		509,500		762,200
Gross Unrealized Losses		(66,000)		(39,600)
Allowance for Credit Losses		(300)		(2,600)
Fair Value		16,945,300		16,332,700
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,996,900		1,317,600
Debt securities, gross unrealized gains		26,100		44,000
Debt securities, gross unrealized losses		(11,200)		(1,500)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		2,011,800		1,360,100
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,482,000		2,489,100
Debt securities, gross unrealized gains		130,900		168,300
Debt securities, gross unrealized losses		(4,400)		(600)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		2,608,500		2,656,800
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		817,000		846,600
Debt securities, gross unrealized gains		17,300		33,400
Debt securities, gross unrealized losses		(5,900)		(500)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		828,400		879,500
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,162,600		3,262,000
Debt securities, gross unrealized gains		190,000		289,700
Debt securities, gross unrealized losses		(14,300)		(3,500)
Debt securities, allowance for credit losses		(300)		(2,500)
Debt securities, fair value		3,338,000		3,545,700
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,202,800		1,268,300
Debt securities, gross unrealized gains		27,000		55,600
Debt securities, gross unrealized losses		(6,000)		(700)
Debt securities, allowance for credit losses		0		(100)
Debt securities, fair value		1,223,800		1,323,100
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,921,900		2,533,600
Debt securities, gross unrealized gains		48,400		78,300
Debt securities, gross unrealized losses		(14,500)		(500)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value	[1]	1,955,800		2,611,400
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		875,200		852,600
Debt securities, gross unrealized gains		36,700		45,300
Debt securities, gross unrealized losses		(2,000)		(7,600)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		909,900		890,300
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	2,983,300		2,328,700
Debt securities, gross unrealized gains	[2]	33,100		47,600
Debt securities, gross unrealized losses	[2]	(7,700)		(24,700)
Debt securities, allowance for credit losses	[2]	0		0
Debt securities, fair value	[3]	3,008,700		2,351,600
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		1,060,400		714,200
Gross Unrealized Gains		0		0
Gross Unrealized Losses		0		0
Allowance for Credit Losses		0		0
Fair Value		$ 1,060,400		$ 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 $1,360.0 million and $878.4 million of collateralized loan obligations as of September 30, 2021 and December 31, 2020, respectively.

[3] Includes $1,360.0 million and $878.4 million of collateralized loan obligations as of September 30, 2021 and December 31, 2020, respectively.