Investments - Realized Gain (Loss) on Investments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 17,469	$ 12,595	$ (705)	$ (5,748)	$ (18,463)	$ 3,482	$ 4,005	$ 293	$ 23,611	$ (10,683)	$ (8,791)
Applicable tax									(6,021)	3,739	3,077
Realized gains (losses) from investments, net of tax									17,590	(6,944)	(5,714)
Loss on redemption on debt
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)									(4,041)	0	0
Fixed maturities available for sale
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)									35,199	(10,645)	(9,479)
Fixed maturities available for sale | Other-than-temporary impairments
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)									(245)	0	0
Other investments
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)									$ (7,302)	$ (38)	$ 688