Fair Value - Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets (Details) - Fixed maturity securities - Income Approach Valuation Technique - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated Fair Value (In millions)	$ 139	$ 106
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	1.00%	2.00%
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	12.00%	40.00%
Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	3.00%	4.00%