Insurance (Guarantees Related to Universal and Variable Life Contracts) (Details) - Universal and Variable Life Contracts - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Secondary Guarantees
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value (1), (3)	$ 9,089	$ 9,036
Net amount at risk (7)	$ 66,762	$ 66,956
Average attained age of policyholders	57 years	56 years
Paid-Up Guarantees
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value (1), (3)	$ 3,171	$ 3,207
Net amount at risk (7)	$ 16,332	$ 16,615
Average attained age of policyholders	63 years	63 years