Business, Basis of Presentation and Significant Accounting Policies - Future Policy Benefits (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Individual and Group Life Insurance Reserves | Minimum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Discount rate (percent)	2.30%
Individual and Group Life Insurance Reserves | Maximum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Discount rate (percent)	7.70%
Fixed annuities and payout contracts without life contingencies
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Credited interest rate maximum on fixed annuities and payout contracts without life contingencies	7.50%	7.50%	7.50%
Future Policy Benefits and Claims Reserves | Minimum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Discount rate (percent)	2.70%
Future Policy Benefits and Claims Reserves | Maximum
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Discount rate (percent)	8.30%