Basis of Presentation and Accounting Policies - Measurement of Credit Losses (Details) - USD ($) $ in Millions	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 1,359	$ 1,104	$ 900	$ 883	$ 883
Total ALLL	1,402	1,145	910	896	897
Accrued expense and other liabilities			75
Total allowance for credit losses			985
Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	818	627	751	737	743
Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	541	477	149	146	140
Commercial and Industrial [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	542	410	551	530	532
Commercial Mortgage Backed Securities [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			143
Construction Loans [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	25	15	22	28	33
Commercial Lease Financing [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	44	43	35	35	36
Residential Mortgage [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			7
Home Equity Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	184	178	31	36	35
Consumer Direct Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	116	97	34	33	30
Commercial Credit Card [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			47
Consumer Indirect Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	48	36	30	22	20
As Reported Under ASC 326
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		1,104
Total ALLL		1,145
Accrued expense and other liabilities		145
Total allowance for credit losses		1,290
As Reported Under ASC 326 | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		627
As Reported Under ASC 326 | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		477
As Reported Under ASC 326 | Commercial and Industrial [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		410
As Reported Under ASC 326 | Commercial Mortgage Backed Securities [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		159
As Reported Under ASC 326 | Construction Loans [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		15
As Reported Under ASC 326 | Commercial Lease Financing [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		43
As Reported Under ASC 326 | Residential Mortgage [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		84
As Reported Under ASC 326 | Home Equity Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		178
As Reported Under ASC 326 | Consumer Direct Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		97
As Reported Under ASC 326 | Commercial Credit Card [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		82
As Reported Under ASC 326 | Consumer Indirect Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		36
Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		204
Total ALLL		235
Accrued expense and other liabilities		70
Total allowance for credit losses		305
Impact of ASC 326 Adoption | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		(124)
Impact of ASC 326 Adoption | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		328
Impact of ASC 326 Adoption | Commercial and Industrial [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		(141)
Impact of ASC 326 Adoption | Commercial Mortgage Backed Securities [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		16
Impact of ASC 326 Adoption | Construction Loans [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		(7)
Impact of ASC 326 Adoption | Commercial Lease Financing [Member] | Commercial Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		8
Impact of ASC 326 Adoption | Residential Mortgage [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		77
Impact of ASC 326 Adoption | Home Equity Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		147
Impact of ASC 326 Adoption | Consumer Direct Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		63
Impact of ASC 326 Adoption | Commercial Credit Card [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		35
Impact of ASC 326 Adoption | Consumer Indirect Loans [Member] | Consumer Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		6
Discontinued Operations [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 43	41	$ 10	$ 13	$ 14
Discontinued Operations [Member] | As Reported Under ASC 326
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		41
Discontinued Operations [Member] | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		$ 31