Change in Carrying Amount of Deferred Acquisition Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
DAC
Balance at beginning of period	$ 33,901	$ 37,500
Capitalization	22	81
Accretion (amortization) expense	(3,931)	(5,207)
Unlocking	215	1,527
Balance at end of period	$ 30,207	$ 33,901