Summary of Quantitative Inputs and Assumptions of Level 3 Assets and Liabilities (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Estimated Fair Value		$ 29,589	$ 57,381
Future policy benefits (embedded derivatives), fair value		$ 38,471	$ 55,143
Own credit risk		0.30%	0.50%
Long-term volatility		25.00%
Long-term volatility		30.00%
Fixed maturity AFS securities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Estimated Fair Value	[1]	$ 4,264	$ 9,215	$ 6,666
Fixed maturity AFS securities | Asset-backed securities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Estimated Fair Value		$ 4,264	$ 9,215
Fixed maturity AFS securities | Asset-backed securities | Level 3
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Valuation Technique		Broker	Broker
GMIB
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Estimated Fair Value	[2]	$ 25,325	$ 48,166	61,426
GMIB | Level 3
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Valuation Technique		Discounted cash flows	Discounted cash flows
GMIB | Level 3 | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Own credit risk		0.30%	0.50%
Long-term volatility		25.00%	25.00%
Long-term volatility		30.00%	30.00%
GMWB
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Future policy benefits (embedded derivatives), fair value	[2]	$ 37,566	$ 54,414	60,618
GMWB | Level 3
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Valuation Technique		Discounted cash flows	Discounted cash flows
GMWB | Level 3 | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Own credit risk		0.30%	0.50%
Long-term volatility		25.00%	25.00%
Long-term volatility		30.00%	30.00%
SALB
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Future policy benefits (embedded derivatives), fair value	[2]	$ 905	$ 729	$ 511

[1] Recorded as a component of fixed maturity AFS securities on the Balance Sheets.

[2] GMWB and SALB are recorded as a component of future policy benefits on the Balance Sheets and GMIB reinsurance is recorded as recoverable of ceded GMIB embedded derivatives, at fair value on the Balance Sheets.