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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
GMDB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Guaranteed benefits incurred	$ 5,096	$ 5,534	$ 15,528	$ 17,199
Guaranteed benefits paid	(3,866)	(4,783)	(13,430)	(16,208)
Unlocking	(7,550)	(9,741)	(5,378)	(26,201)
Increase (decrease) in liability balance	(6,320)	(8,990)	(3,280)	(25,210)
GMIB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Guaranteed benefits incurred	2,032	2,649	6,333	8,452
Guaranteed benefits paid	(418)	(270)	(1,095)	(2,288)
Unlocking	(4,412)	(6,100)	(13,979)	(24,551)
Increase (decrease) in liability balance	(2,798)	(3,721)	(8,741)	(18,387)
GMWB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Change in fair value reserves	(5,875)	(7,910)	(12,380)	(13,565)
Increase (decrease) in liability balance	$ (5,875)	$ (7,910)	$ (12,380)	$ (13,565)