Increases (Decreases) 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
	Mar. 31, 2018	Mar. 31, 2017
GMDB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Guaranteed benefits incurred	$ 5,262	$ 5,969
Guaranteed benefits paid	(4,814)	(4,634)
Unlocking	3,088	(13,657)
Increase (decrease) in liability balance	3,536	(12,322)
GMIB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Guaranteed benefits incurred	2,281	3,061
Guaranteed benefits paid	(411)	(1,294)
Unlocking	781	(13,105)
Increase (decrease) in liability balance	2,651	(11,338)
GMWB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Change in fair value reserves	(3,660)	(6,748)
Increase (decrease) in liability balance	$ (3,660)	$ (6,748)