Loans and Allowance for Loan Losses - Average Investment in Impaired Loans and Interest Income Recognized for Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Loan impairment
Average Recorded Investment	$ 834	$ 771
Recognized Interest Income	11	19
Commercial
Loan impairment
Average Recorded Investment	557	458
Recognized Interest Income	6	14
Consumer
Loan impairment
Average Recorded Investment	277	313
Recognized Interest Income	5	5
Commercial and industrial \| Commercial Loans Portfolio Segment [Member]
Loan impairment
Average Recorded Investment	430	293
Recognized Interest Income	3	2
Construction \| Commercial Loans Portfolio Segment [Member]
Loan impairment
Average Recorded Investment	62	128
Recognized Interest Income	1	2
Home equity and other consumer loans \| Consumer Loans
Loan impairment
Average Recorded Investment	33	41
Recognized Interest Income	2	1
Residential mortgage \| Consumer Loans
Loan impairment
Average Recorded Investment	244	272
Recognized Interest Income	3	4
Commercial mortgage \| Commercial Loans Portfolio Segment [Member]
Loan impairment
Average Recorded Investment	65	37
Recognized Interest Income	$ 2	$ 10

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

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