Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 15,993,207	$ 15,727,143
Debt securities, gross unrealized gains		106,200	433,800
Debt securities, gross unrealized losses		(539,700)	(98,800)
Debt securities, allowance for credit losses		(1,084)	(502)	$ (600)	$ (2,600)
Debt securities, fair value		15,558,624	16,061,560
Amortized Cost or Cost		17,299,000	16,869,400
Gross Unrealized Gains		106,200	433,800
Gross Unrealized Losses		(539,700)	(98,800)
Allowance for Credit Losses		(1,100)	(500)
Fair Value		16,864,400	17,203,900
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,649,900	2,039,700
Debt securities, gross unrealized gains		4,000	27,900
Debt securities, gross unrealized losses		(60,900)	(16,900)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value		1,593,000	2,050,700
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,673,200	2,412,700
Debt securities, gross unrealized gains		39,200	127,300
Debt securities, gross unrealized losses		(70,500)	(4,100)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value		2,641,900	2,535,900
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		844,100	850,800
Debt securities, gross unrealized gains		1,700	12,000
Debt securities, gross unrealized losses		(27,800)	(7,900)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value		818,000	854,900
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,421,900	3,336,900
Debt securities, gross unrealized gains		44,400	159,200
Debt securities, gross unrealized losses		(115,000)	(18,300)
Debt securities, allowance for credit losses		(500)	(500)
Debt securities, fair value		3,350,800	3,477,300
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,232,600	1,216,600
Debt securities, gross unrealized gains		2,800	19,900
Debt securities, gross unrealized losses		(48,400)	(9,600)
Debt securities, allowance for credit losses		(600)	0
Debt securities, fair value		1,186,400	1,226,900
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,922,000	1,993,600
Debt securities, gross unrealized gains		9,100	37,200
Debt securities, gross unrealized losses		(99,200)	(21,800)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value	[1]	1,831,900	2,009,000
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,068,400	879,800
Debt securities, gross unrealized gains		1,900	28,300
Debt securities, gross unrealized losses		(21,600)	(2,200)
Debt securities, allowance for credit losses		0	0
Debt securities, fair value		1,048,700	905,900
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	3,181,100	2,997,000
Debt securities, gross unrealized gains	[2]	3,100	22,000
Debt securities, gross unrealized losses	[2]	(96,300)	(18,000)
Debt securities, allowance for credit losses	[2]	0	0
Debt securities, fair value	[2],[3]	3,087,900	3,001,000
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		1,305,800	1,142,300
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Allowance for Credit Losses		0	0
Fair Value		$ 1,305,800	$ 1,142,300

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

[2] Includes $1,281.5 million and $1,320.4 million of collateralized loan obligations as of March 31, 2022 and December 31, 2021, respectively.

[3] Includes $1,281.5 million and $1,320.4 million of collateralized loan obligations as of March 31, 2022 and December 31, 2021, respectively.