Short-term Borrowings Short-term Borrowings (Summary of Short-term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at year-end	$ 71	$ 44
Federal Funds Purchased and Securities Sold Under Agreements to Repurchase
Short-term Debt [Line Items]
Amount outstanding at year-end	$ 71	$ 44	$ 10
Weighted average interest rate at year-end	1.50%	2.39%	1.43%
Maximum month-end balance during the year	$ 835	$ 182	$ 41
Average balance outstanding during the year	$ 113	$ 59	$ 20
Weighted average interest rate during the year	2.28%	1.91%	1.02%
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	$ 1,200	$ 250	$ 1,024
Average balance outstanding during the year	$ 256	$ 3	$ 257
Weighted average interest rate during the year	2.44%	1.75%	1.15%