Changes in Variable Annuity Guaranteed Minimum Death Benefits, Guaranteed Minimum Income Benefits and Guaranteed Minimum Withdrawal Benefits Liabilities (Detail) - Variable Annuity Contracts - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
GMDB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Guaranteed benefits incurred	$ 5,534	$ 6,429	$ 17,199	$ 19,295
Guaranteed benefits paid	(4,783)	(3,060)	(16,208)	(16,357)
Unlocking	(9,741)	(22,768)	(26,201)	(17,928)
Total	(8,990)	(19,399)	(25,210)	(14,990)
GMIB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Guaranteed benefits incurred	2,649	3,648	8,452	10,413
Guaranteed benefits paid	(270)	(300)	(2,288)	(1,706)
Unlocking	(6,100)	(8,195)	(24,551)	(3,015)
Total	(3,721)	(4,847)	(18,387)	5,692
GMWB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Change in fair value reserves	(7,910)	(7,159)	(13,565)	16,713
Total	$ (7,910)	$ (7,159)	$ (13,565)	$ 16,713