Amortized Cost or Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 14,012,407	$ 11,895,850
Debt securities, gross unrealized gains		505,800	98,900
Debt securities, gross unrealized losses		(29,800)	(170,800)
Debt securities, fair value		14,488,289	11,823,968
Amortized Cost or Cost		14,980,800	12,789,700
Gross Unrealized Gains		505,800	98,900
Gross Unrealized Losses		(29,800)	(170,800)
Fair Value		15,456,800	12,717,800
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,262,800	1,042,400
Debt securities, gross unrealized gains		26,900	2,400
Debt securities, gross unrealized losses		(1,500)	(22,400)
Debt securities, fair value		1,288,200	1,022,400
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,218,300	2,177,500
Debt securities, gross unrealized gains		130,400	44,400
Debt securities, gross unrealized losses			(7,200)
Debt securities, fair value		2,348,700	2,214,700
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		728,200	939,000
Debt securities, gross unrealized gains		29,100	12,300
Debt securities, gross unrealized losses		(400)	(3,400)
Debt securities, fair value		756,900	947,900
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,174,300	2,431,400
Debt securities, gross unrealized gains		155,200	13,200
Debt securities, gross unrealized losses		(3,500)	(59,300)
Debt securities, fair value		3,326,000	2,385,300
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,417,200	1,363,000
Debt securities, gross unrealized gains		41,700	9,100
Debt securities, gross unrealized losses		(4,800)	(18,800)
Debt securities, fair value		1,454,100	1,353,300
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,881,500	1,392,400
Debt securities, gross unrealized gains		55,400	10,300
Debt securities, gross unrealized losses		(1,200)	(14,800)
Debt securities, fair value	[1]	1,935,700	1,387,900
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		786,500	536,900
Debt securities, gross unrealized gains		35,100	2,800
Debt securities, gross unrealized losses		(100)	(6,400)
Debt securities, fair value		821,500	533,300
Other Asset Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[2]	2,543,500	2,013,300
Debt securities, gross unrealized gains	[2]	32,000	4,400
Debt securities, gross unrealized losses	[2]	(18,300)	(38,500)
Debt securities, fair value	[2]	2,557,200	1,979,200
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		968,500	893,800
Fair Value		$ 968,500	$ 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 $842.1 million and $1,266.9 million of collateralized loan obligations as of September 30, 2019 and December 31, 2018, respectively.