Loans and Allowance for Loan Losses - Purchased Credit-Impaired Loans Segment (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Commercial
Aging of loans
Total loans	$ 45,745	$ 43,135
Commercial | Commercial and industrial
Aging of loans
Total loans	27,339	26,168
Commercial | Commercial mortgage
Aging of loans
Total loans	16,895	15,354
Commercial | Construction
Aging of loans
Total loans	1,511	1,613
Pass | Commercial
Aging of loans
Total loans	44,178	41,871
Pass | Commercial | Commercial and industrial
Aging of loans
Total loans	26,210	25,191
Pass | Commercial | Commercial mortgage
Aging of loans
Total loans	16,569	15,138
Pass | Commercial | Construction
Aging of loans
Total loans	1,399	1,542
Special Mention | Commercial
Aging of loans
Total loans	800	468
Special Mention | Commercial | Commercial and industrial
Aging of loans
Total loans	636	355
Special Mention | Commercial | Commercial mortgage
Aging of loans
Total loans	114	105
Special Mention | Commercial | Construction
Aging of loans
Total loans	50	8
Classified | Commercial
Aging of loans
Total loans	767	796
Classified | Commercial | Commercial and industrial
Aging of loans
Total loans	493	622
Classified | Commercial | Commercial mortgage
Aging of loans
Total loans	212	111
Classified | Commercial | Construction
Aging of loans
Total loans	62	63
Consumer Portfolio Segment [Member]
Aging of loans
Loans not covered	$ 3	$ 6