Derivatives (Credit Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Maximum Length of Time Hedged in Cash Flow Hedge	2 years	2 years
Estimated Fair Value of Credit Default Swaps	$ 31	$ 40
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,906	$ 1,878
Weighted Average Years to Maturity	4 years 4 months	4 years 1 month
Aaa/Aa/A | Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 11	$ 12
Maximum Amount of Future Payments under Credit Default Swaps	$ 607	$ 558
Weighted Average Years to Maturity	2 years 8 months	2 years 9 months
Baa | Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 20	$ 28
Maximum Amount of Future Payments under Credit Default Swaps	$ 1,299	$ 1,295
Weighted Average Years to Maturity	5 years	4 years 8 months
Ba | Aaa/Aa/A
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 0	$ 0
Maximum Amount of Future Payments under Credit Default Swaps	$ 0	$ 25
Weighted Average Years to Maturity	0 years	4 years 6 months