Revenue from Contracts with Customers (Details) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Mar. 31, 2019 Segment	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 Segment
Disaggregation of Revenue [Line Items]
Number of operating segments | Segment		2				2
Total revenue from contracts with customers	$ 341		$ 334	$ 966	$ 981
Noninterest income	650		609	1,808	1,870
Contract assets	0		0	0	0
Contract liabilities	0		0	0	0
Trust And Investment Services Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	105		106	313	318
Investment Banking And Debt Placement Fees [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	78		70	188	185
Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	86		85	251	265
Cards And Payments Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	69		67	204	197
Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3		6	10	16
Noninterest income	277		230	752	677
Operating Segments [Member] | Consumer Bank [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	193		192	567	575
Operating Segments [Member] | Consumer Bank [Member] | Trust And Investment Services Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	89		89	265	264
Operating Segments [Member] | Consumer Bank [Member] | Investment Banking And Debt Placement Fees [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0		0	0	0
Operating Segments [Member] | Consumer Bank [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	59		57	169	181
Operating Segments [Member] | Consumer Bank [Member] | Cards And Payments Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	42		40	123	115
Operating Segments [Member] | Consumer Bank [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3		6	10	15
Operating Segments [Member] | Commercial Bank [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	148		142	399	406
Operating Segments [Member] | Commercial Bank [Member] | Trust And Investment Services Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	16		17	48	54
Operating Segments [Member] | Commercial Bank [Member] | Investment Banking And Debt Placement Fees [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	78		70	188	185
Operating Segments [Member] | Commercial Bank [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	27		28	82	84
Operating Segments [Member] | Commercial Bank [Member] | Cards And Payments Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	27		27	81	82
Operating Segments [Member] | Commercial Bank [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0		0	0	1
Other [Member]
Disaggregation of Revenue [Line Items]
Noninterest income	$ 32		$ 45	$ 90	$ 212