Short-Term Borrowings - Components of Short-Term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Balance at year end, Securities Sold Under Repurchase Agreements	$ 0	$ 0
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Balance at year end, Federal Funds Purchase	0	3	$ 1,005
Average during the year	537	128	44
Maximum month-end balance	$ 3,197	$ 2,331	$ 1,005
Weighted-average rate during the year	1.68%	0.72%	0.68%
Weighted-average rate at December 31	0.00%	0.00%	0.55%
Securities Sold under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Average during the year	$ 391	$ 389	$ 443
Maximum month-end balance	$ 614	$ 472	$ 684
Weighted-average rate during the year	0.09%	0.08%	0.04%
Weighted-average rate at December 31	0.09%	0.08%	0.07%
Balance at year end, Securities Sold Under Repurchase Agreements	$ 319	$ 374	$ 497
Other Short Term Borrowings [Member]
Short-term Debt [Line Items]
Average during the year	915	1,140	852
Maximum month-end balance	$ 1,133	$ 1,242	$ 872
Weighted-average rate during the year	2.34%	1.34%	1.18%
Weighted-average rate at December 31	2.92%	2.01%	1.11%
Balance at year end, Other Short-Term Borrowings	$ 544	$ 634	$ 808