Variable Interest Entities (Aggregate Assets, Liabilities and Maximum Exposure to Loss) (Details) - Variable Interest Entity, Not Primary Beneficiary [Member] - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Aggregate assets	$ 316	$ 249
Aggregate liabilities	169	135
Maximum exposure to loss	335	269
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Aggregate assets	253	189
Aggregate liabilities	169	135
Maximum exposure to loss	253	189
Other CRA Investments [Member]
Variable Interest Entity [Line Items]
Aggregate assets	63	60
Aggregate liabilities	0	0
Maximum exposure to loss	$ 82	$ 80