Investments - Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 15,694,482		$ 15,727,143
Debt securities, gross unrealized gains		26,500		433,800
Debt securities, gross unrealized losses		(1,061,000)		(98,800)
Debt securities, allowance for credit losses		(8,679)	$ (1,100)	(502)	$ (400)	$ (600)	$ (2,600)
Debt securities, fair value		14,651,254		16,061,560
Amortized Cost or Cost		17,318,900		16,869,400
Gross Unrealized Gains		26,500		433,800
Gross Unrealized Losses		(1,061,000)		(98,800)
Allowance for Credit Losses		(8,700)		(500)
Fair Value		16,275,700		17,203,900
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,637,200		2,039,700
Debt securities, gross unrealized gains		700		27,900
Debt securities, gross unrealized losses		(99,400)		(16,900)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		1,538,500		2,050,700
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,764,100		2,412,700
Debt securities, gross unrealized gains		13,900		127,300
Debt securities, gross unrealized losses		(151,200)		(4,100)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		2,626,800		2,535,900
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		790,300		850,800
Debt securities, gross unrealized gains		200		12,000
Debt securities, gross unrealized losses		(46,300)		(7,900)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		744,200		854,900
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,414,600		3,336,900
Debt securities, gross unrealized gains		6,700		159,200
Debt securities, gross unrealized losses		(256,600)		(18,300)
Debt securities, allowance for credit losses		(7,500)		(500)
Debt securities, fair value		3,157,200		3,477,300
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,180,100		1,216,600
Debt securities, gross unrealized gains		200		19,900
Debt securities, gross unrealized losses		(94,000)		(9,600)
Debt securities, allowance for credit losses		(1,200)		0
Debt securities, fair value		1,085,100		1,226,900
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,837,300		1,993,600
Debt securities, gross unrealized gains		3,900		37,200
Debt securities, gross unrealized losses		(172,500)		(21,800)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value	[1]	1,668,700		2,009,000
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		980,900		879,800
Debt securities, gross unrealized gains		100		28,300
Debt securities, gross unrealized losses		(55,300)		(2,200)
Debt securities, allowance for credit losses		0		0
Debt securities, fair value		925,700		905,900
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	3,090,000		2,997,000
Debt securities, gross unrealized gains	[2]	800		22,000
Debt securities, gross unrealized losses	[2]	(185,700)		(18,000)
Debt securities, allowance for credit losses	[2]	0		0
Debt securities, fair value	[2],[3]	2,905,100		3,001,000
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		1,624,400		1,142,300
Gross Unrealized Gains		0		0
Gross Unrealized Losses		0		0
Allowance for Credit Losses		0		0
Fair Value		$ 1,624,400		$ 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 $1,229.6 million and $1,320.4 million of collateralized loan obligations as of June 30, 2022 and December 31, 2021, respectively.

[3] Includes $1,229.6 million and $1,320.4 million of collateralized loan obligations as of June 30, 2022 and December 31, 2021, respectively.