Schedule of Total Investment Return (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
Net Investment Income [Line Items]
Net investment income	$ 8,880	$ 8,795	$ 6,562	$ 9,746	$ 8,375	$ 8,852	$ 9,141	$ 8,241	$ 33,983	$ 34,609	$ 28,821
Net realized investment gains(losses)	$ 30,778	$ 1,928	$ (3,492)	$ (7,493)	$ 3,842	$ (10,778)	$ 6,532	$ (2,970)	21,721	(3,374)	35,860
Change in unrealized holding gains and losses									(8,240)	(25,673)	(45,861)
Net realized and unrealized investment returns									13,481	(29,047)	(10,001)
Total investment return									$ 47,464	$ 5,562	$ 18,820
Total investment return %									3.10%	0.30%	1.20%
Average investment portfolio									$ 1,507,184	$ 1,752,785	$ 1,533,104