Variable Interest Entities (Aggregate Assets, Liabilities and Maximum Exposure to Loss) (Details) - Variable Interest Entity, Not Primary Beneficiary [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Aggregate assets	$ 407	$ 373
Aggregate liabilities	217	203
Maximum exposure to loss	462	429
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Aggregate assets	340	304
Aggregate liabilities	217	203
Maximum exposure to loss	340	304
Other CRA Investments [Member]
Variable Interest Entity [Line Items]
Aggregate assets	67	69
Aggregate liabilities	0	0
Maximum exposure to loss	$ 122	$ 125