Insurance (Guarantees Related to Annuity Contracts) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Variable Annuity Guarantees: | Guaranteed Death Benefits
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value	$ 64,728	$ 66,724
Separate account value (1)	43,714	45,431
Net amount at risk	$ 1,587	$ 1,238
Average attained age of contractholders	66 years	65 years
Variable Annuity Guarantees: | Guaranteed Annuitization Benefits
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value	$ 25,169	$ 26,223
Separate account value (1)	23,302	24,336
Net amount at risk	$ 510	$ 525
Average attained age of contractholders	66 years	65 years
Other Annuity Guarantees: | Guaranteed Annuitization Benefits
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value	$ 1,437	$ 1,424
Net amount at risk	$ 559	$ 569
Average attained age of contractholders	50 years	50 years