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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
GMDB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Beginning Balance	$ 112,983	$ 131,378
Guaranteed benefits incurred	22,623	25,408
Guaranteed benefits paid	(22,570)	(23,432)
Unlocking	(34,164)	(20,371)
Ending Balance	78,872	112,983
GMIB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Beginning Balance	96,702	95,048
Guaranteed benefits incurred	10,972	13,750
Guaranteed benefits paid	(2,892)	(2,050)
Unlocking	(32,762)	(10,046)
Ending Balance	72,020	96,702
GMWB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Beginning Balance	54,414	60,618
Change in fair value reserves	(16,848)	(6,204)
Ending Balance	$ 37,566	$ 54,414