Revenue from Contracts with Customers (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) Segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 Segment
Disaggregation of Revenue [Line Items]
Number of operating segments | Segment	2		2
Total revenue from contracts with customers	$ 299	$ 320
Noninterest income	536	601
Contract assets	0	0
Contract liabilities	0	0
Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	100	119
Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	45	47
Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	87	89
Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	64	60
Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3	5
Noninterest income	215	234
Operating Segments [Member] | Community Banking [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	186	199
Operating Segments [Member] | Community Banking [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	85	99
Operating Segments [Member] | Community Banking [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0	0
Operating Segments [Member] | Community Banking [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	60	61
Operating Segments [Member] | Community Banking [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	38	34
Operating Segments [Member] | Community Banking [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3	5
Operating Segments [Member] | Key Corporate Bank [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	113	121
Operating Segments [Member] | Key Corporate Bank [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	15	20
Operating Segments [Member] | Key Corporate Bank [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	45	47
Operating Segments [Member] | Key Corporate Bank [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	27	28
Operating Segments [Member] | Key Corporate Bank [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	26	26
Operating Segments [Member] | Key Corporate Bank [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0	0
Other [Member]
Disaggregation of Revenue [Line Items]
Noninterest income	$ 22	$ 47