Short-term Borrowings Short-term Borrowings (Summary of Short-term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding at year-end	$ 10	$ 25
Federal Funds Purchased and Securities Sold Under Agreements to Repurchase
Short-term Debt [Line Items]
Amount outstanding at year-end	$ 10	$ 25	$ 23
Weighted average interest rate at year-end	1.43%	0.54%	0.38%
Maximum month-end balance during the year	$ 41	$ 25	$ 109
Average balance outstanding during the year	$ 20	$ 15	$ 93
Weighted average interest rate during the year	1.02%	0.47%	0.05%
Other Short-term Borrowings
Short-term Debt [Line Items]
Amount outstanding at year-end	$ 0	$ 0	$ 0
Weighted average interest rate at year-end	0.00%	0.00%	0.00%
Maximum month-end balance during the year	$ 1,024	$ 501	$ 0
Average balance outstanding during the year	$ 257	$ 123	$ 0
Weighted average interest rate during the year	1.15%	0.45%	0.00%