Guaranteed Benefit Features - Assumptions and Methodology Used to Determine Additional Reserves (Details) - investment_performance_scenarios	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Number of investment performance scenarios	1,000	1,000
Variable Life and Universal Life | Paid-up Guarantees
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Weighted average attained age	0 years	0 years
GMDB
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Investment blended rate of return (percent)	7.80%	7.80%
Volatility rate (percent)	13.00%	14.20%
Discount rate (percent)	5.50%	5.50%
Weighted average attained age	68 years	68 years
GMIB
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Investment blended rate of return (percent)	8.10%	8.10%
Volatility rate (percent)	14.30%	14.20%
Discount rate (percent)	5.50%	5.50%
Weighted average attained age	62 years	62 years
GMAB
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Period of implied volatility (years)	5 years	5 years
GMWB
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Period of implied volatility (years)	5 years	5 years
GMWBL
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Period of implied volatility (years)	5 years	5 years
Weighted average attained age	71 years	70 years