Insurance (Guarantees Related to Universal and Variable Life Contracts) (Details) - Secondary Guarantees - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Universal Life [Member]
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value (3)	$ 6,187	$ 6,244
Net amount at risk	$ 74,755	$ 75,304
Average attained age of policyholders	64 years	64 years
Variable Life [Member]
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value (3)	$ 1,018	$ 1,021
Net amount at risk	$ 13,829	$ 13,848
Average attained age of policyholders	44 years	44 years