Loans and Allowance for Loan Losses - Pre and Post Modification Outstanding Recorded Investment Amounts of TDRs (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	$ 310	$ 180
Post-Modification Outstanding Recorded Investment	308	180
Commercial \| Commercial mortgage
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	129	4
Post-Modification Outstanding Recorded Investment	125	4
Consumer \| Other Consumer Loans [Member]
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	0	0
Post-Modification Outstanding Recorded Investment	0	0
Commercial Portfolio Segment [Member]
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	299	169
Post-Modification Outstanding Recorded Investment	297	169
Commercial Portfolio Segment [Member] \| Commercial and industrial
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	170	165
Post-Modification Outstanding Recorded Investment	172	165
Consumer Portfolio Segment [Member]
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	11	11
Post-Modification Outstanding Recorded Investment	11	11
Consumer Portfolio Segment [Member] \| residential mortgage and home equity [Member]
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	11	11
Post-Modification Outstanding Recorded Investment	$ 11	$ 11

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Amount, after troubled debt restructuring, of modified financing receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

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Amount, before troubled debt restructuring, of financing receivable to be modified.

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