Reclassifications of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains		$ (32,921)	$ (27,221)	$ (101,840)	$ (117,126)
Other than temporary impairment losses		6,131	11,729	13,095	38,216
Income taxes		(212,379)	48,328	(116,368)	162,274
Total reclassifications		(17,400)	(10,100)	(57,700)	(42,700)
Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications				(57,700)	(42,700)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains	[1]	(32,900)	(27,200)	(101,800)	(103,900)
Other than temporary impairment losses		6,100	11,700	13,100	38,200
Income taxes		$ 9,400	$ 5,400	$ 31,000	$ 23,000

[1] Net realized capital gains for the nine months ended September 30, 2016 exclude the Jazwares Remeasurement Gain of $13.2 million.