Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Estimated fair value	$ 41,606	$ 43,536
Fixed maturity securities | Discounted cash flow
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Estimated fair value	$ 198	$ 136
Fixed maturity securities | Discounted cash flow | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Credit spread	1.00%	1.00%
Fixed maturity securities | Discounted cash flow | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Credit spread	12.00%	12.00%
Fixed maturity securities | Discounted cash flow | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Credit spread	2.00%	3.00%