Variable Annuity Contracts Containing Guaranteed Benefits (Detail) - Variable Annuity Contracts - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
GMDB
Net Amount at Risk by Product and Guarantee [Line Items]
Net amount at risk	[1]	$ 552,405	$ 741,564
Average attained age of contract owners		74 years	74 years
GMIB
Net Amount at Risk by Product and Guarantee [Line Items]
Net amount at risk	[1]	$ 19,440	$ 27,017
Average attained age of contract owners		68 years	68 years
GMWB
Net Amount at Risk by Product and Guarantee [Line Items]
Net amount at risk	[1]	$ 4,813	$ 8,106
Average attained age of contract owners		77 years	76 years

[1] Net amount at risk for GMDB is defined as the current GMDB in excess of the contract owners’ account balance at the Balance Sheet date. Net amount at risk for GMIB is defined as the present value of the minimum guaranteed annuity payments available to the contract owner in excess of the contract owners’ account balance at the Balance Sheet date. Net amount at risk for GMWB is defined as the difference between the maximum amount payable under the guarantee and the contract owners’ account balance at the Balance Sheet date.