Investments - Realized Gain (Loss) on Investments (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
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ (18,463)	$ 3,482	$ 4,005	$ 293	$ (16,663)	$ 5,140	$ 2,613	$ 119	$ (10,683)	$ (8,791)	$ 23,548
Applicable tax									3,739	3,077	(8,242)
Realized gains (losses) from investments, net of tax									(6,944)	(5,714)	15,306
Loss on redemption on debt
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)									0	0	(258)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)									(10,645)	(9,479)	23,170
Other investments
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)									$ (38)	$ 688	$ 636