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	Mar. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated fair value	$ 133	$ 136
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	1.00%	1.00%
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	12.00%	12.00%
Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	3.00%	3.00%