Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other net realized investment (gains) losses									$ 28,454	$ 3,961	$ 36,361
Other than temporary impairment losses on investments									(6,733)	(7,335)	(501)
Income before income taxes	$ 46,535	$ 10,598	$ (11,468)	$ 1,953	$ 29,463	$ (9,727)	$ 9,772	$ 3,238	47,618	32,746	71,194
Income tax expense (benefit)									2,250	8,723	(8,338)
Net income	$ 38,373	$ 9,535	$ (5,165)	$ 7,125	$ 27,304	$ (3,746)	$ 11,117	$ 6,794	49,868	41,469	$ 62,856
Total reclassifications, net of tax									(14,809)	(1,985)
Unrealized Gains and Losses on Available for Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax									(15,128)	(2,382)
Foreign Currency Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax									319	397
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains and Losses on Available for Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other net realized investment (gains) losses									(30,055)	(11,559)
Other than temporary impairment losses on investments									6,733	7,335
Income before income taxes									(23,322)	(4,224)
Income tax expense (benefit)									8,194	1,842
Net income									(15,128)	(2,382)
Reclassification out of Accumulated Other Comprehensive Income | Foreign Currency Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other net realized investment (gains) losses									491	610
Income tax expense (benefit)									(172)	(213)
Net income									$ 319	$ 397