Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 13,798,576	$ 11,895,850
Debt securities, gross unrealized gains		445,800	98,900
Debt securities, gross unrealized losses		(32,800)	(170,800)
Debt securities, fair value		14,211,745	11,823,968
Amortized Cost or Cost		14,713,500	12,789,700
Gross Unrealized Gains		445,800	98,900
Gross Unrealized Losses		(32,800)	(170,800)
Fair Value		15,126,500	12,717,800
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,198,500	1,042,400
Debt securities, gross unrealized gains		19,300	2,400
Debt securities, gross unrealized losses		(2,800)	(22,400)
Debt securities, fair value		1,215,000	1,022,400
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,190,500	2,177,500
Debt securities, gross unrealized gains		118,700	44,400
Debt securities, gross unrealized losses		(1,300)	(7,200)
Debt securities, fair value		2,307,900	2,214,700
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		675,900	939,000
Debt securities, gross unrealized gains		16,400	12,300
Debt securities, gross unrealized losses		(1,600)	(3,400)
Debt securities, fair value		690,700	947,900
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,206,200	2,431,400
Debt securities, gross unrealized gains		155,100	13,200
Debt securities, gross unrealized losses		(2,300)	(59,300)
Debt securities, fair value		3,359,000	2,385,300
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,348,100	1,363,000
Debt securities, gross unrealized gains		32,200	9,100
Debt securities, gross unrealized losses		(2,900)	(18,800)
Debt securities, fair value		1,377,400	1,353,300
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,785,100	1,392,400
Debt securities, gross unrealized gains		56,100	10,300
Debt securities, gross unrealized losses		(800)	(14,800)
Debt securities, fair value	[1]	1,840,400	1,387,900
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		849,900	536,900
Debt securities, gross unrealized gains		23,200	2,800
Debt securities, gross unrealized losses		(1,400)	(6,400)
Debt securities, fair value		871,700	533,300
Other Asset Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	2,544,500	2,013,300
Debt securities, gross unrealized gains	[2]	24,800	4,400
Debt securities, gross unrealized losses	[2]	(19,700)	(38,500)
Debt securities, fair value	[2]	2,549,600	1,979,200
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		914,800	893,800
Fair Value		$ 914,800	$ 893,800

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

[2] Includes $834.2 million and $1,266.9 million of collateralized loan obligations as of December 31, 2019 and 2018, respectively.