Interest -Crediting Rates for Fixed Rate Products (Detail) - Deferred Annuities	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Liability for Future Policy Benefit, by Product Segment [Line Items]
Interest-sensitive life products, interest-crediting rates	4.00%	4.00%
Minimum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Interest-sensitive deferred annuities, interest-crediting rates	0.25%	0.25%
Maximum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Interest-sensitive deferred annuities, interest-crediting rates	4.42%	4.90%