Insurance (Guarantees Related to Annuity Contracts) (Details) - Variable Annuity Guarantees - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Guaranteed Death Benefits
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value	$ 101,598	$ 105,061
Separate account value	96,628	100,043
Net amount at risk	$ 6,173	$ 5,200
Average attained age of contract holders	68 years	68 years
Guaranteed Annuitization Benefits
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value	$ 57,518	$ 59,691
Separate account value	56,364	58,511
Net amount at risk	$ 4,432	$ 2,330
Average attained age of contract holders	68 years	68 years