Change in Carrying Amount of Deferred Sales Inducements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
DSI
Balance at beginning of period	$ 7,708	$ 8,518
Capitalization	2	33
Accretion (amortization) expense	(892)	(1,190)
Unlocking	49	347
Balance at end of period	$ 6,867	$ 7,708