Variable Interest Entities (Aggregate Assets, Liabilities and Maximum Exposure to Loss) (Details) - Variable Interest Entity, Not Primary Beneficiary [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Aggregate assets	$ 373	$ 249
Aggregate liabilities	203	135
Maximum exposure to loss	429	269
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Aggregate assets	304	189
Aggregate liabilities	203	135
Maximum exposure to loss	304	189
Other CRA Investments [Member]
Variable Interest Entity [Line Items]
Aggregate assets	69	60
Aggregate liabilities	0	0
Maximum exposure to loss	$ 125	$ 80