Loans and ACL - Summary of the Primary Reason Loan Modifications Were Classified as TDRs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Newly Designated TDRs [Member] | Commercial | Commercial and industrial
Financing Receivable, Modifications [Line Items]
ALLL Impact	$ 3	$ 3	$ 2
Newly Designated TDRs [Member] | Commercial | CRE
Financing Receivable, Modifications [Line Items]
ALLL Impact	1	0	1
Newly Designated TDRs [Member] | Commercial | Rate | Commercial and industrial
Financing Receivable, Modifications [Line Items]
TDRs	79	105	68
Newly Designated TDRs [Member] | Commercial | Rate | CRE
Financing Receivable, Modifications [Line Items]
TDRs	14	12	11
Newly Designated TDRs [Member] | Commercial | Structure | Commercial and industrial
Financing Receivable, Modifications [Line Items]
TDRs	101	96	31
Newly Designated TDRs [Member] | Commercial | Structure | CRE
Financing Receivable, Modifications [Line Items]
TDRs	10	16	26
Newly Designated TDRs [Member] | Retail | Residential mortgage
Financing Receivable, Modifications [Line Items]
ALLL Impact	25	28	16
Newly Designated TDRs [Member] | Retail | Direct
Financing Receivable, Modifications [Line Items]
ALLL Impact	0	0	4
Newly Designated TDRs [Member] | Retail | Indirect
Financing Receivable, Modifications [Line Items]
ALLL Impact	21	21	18
Newly Designated TDRs [Member] | Retail | Rate | Residential mortgage
Financing Receivable, Modifications [Line Items]
TDRs	357	431	230
Newly Designated TDRs [Member] | Retail | Rate | Direct
Financing Receivable, Modifications [Line Items]
TDRs	10	14	12
Newly Designated TDRs [Member] | Retail | Rate | Indirect
Financing Receivable, Modifications [Line Items]
TDRs	192	169	129
Newly Designated TDRs [Member] | Retail | Structure | Residential mortgage
Financing Receivable, Modifications [Line Items]
TDRs	46	53	34
Newly Designated TDRs [Member] | Retail | Structure | Direct
Financing Receivable, Modifications [Line Items]
TDRs	3	1	2
Newly Designated TDRs [Member] | Retail | Structure | Indirect
Financing Receivable, Modifications [Line Items]
TDRs	6	7	9
Newly Designated TDRs [Member] | Revolving credit
Financing Receivable, Modifications [Line Items]
ALLL Impact	4	4	4
Newly Designated TDRs [Member] | Revolving credit | Rate
Financing Receivable, Modifications [Line Items]
TDRs	19	17	16
Newly Designated TDRs [Member] | Revolving credit | Structure
Financing Receivable, Modifications [Line Items]
TDRs	0	0	0
Re-Modification of Previously Designated TDRs [Member]
Financing Receivable, Modifications [Line Items]
ALLL Impact	0	0	0
Re-Modification of Previously Designated TDRs [Member] | Rate
Financing Receivable, Modifications [Line Items]
TDRs	176	79	88
Re-Modification of Previously Designated TDRs [Member] | Structure
Financing Receivable, Modifications [Line Items]
TDRs	$ 44	$ 46	$ 34