Loans and Allowance for Loan Losses - Purchased Credit-Impaired Loans Segment (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Commercial
Aging of loans
Total loans	$ 43,135	$ 40,909
Commercial | Commercial and industrial
Aging of loans
Total loans	26,168	24,814
Commercial | Commercial mortgage
Aging of loans
Total loans	15,354	14,320
Commercial | Construction
Aging of loans
Total loans	1,613	1,775
Pass | Commercial
Aging of loans
Total loans	41,871	39,345
Pass | Commercial | Commercial and industrial
Aging of loans
Total loans	25,191	23,632
Pass | Commercial | Commercial mortgage
Aging of loans
Total loans	15,138	14,081
Pass | Commercial | Construction
Aging of loans
Total loans	1,542	1,632
Special Mention | Commercial
Aging of loans
Total loans	468	530
Special Mention | Commercial | Commercial and industrial
Aging of loans
Total loans	355	435
Special Mention | Commercial | Commercial mortgage
Aging of loans
Total loans	105	80
Special Mention | Commercial | Construction
Aging of loans
Total loans	8	15
Classified | Commercial
Aging of loans
Total loans	796	1,034
Classified | Commercial | Commercial and industrial
Aging of loans
Total loans	622	747
Classified | Commercial | Commercial mortgage
Aging of loans
Total loans	111	159
Classified | Commercial | Construction
Aging of loans
Total loans	63	128
Consumer Portfolio Segment [Member]
Aging of loans
Loans not covered	$ 6	$ 8