Revenue from Contracts with Customers (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 1,330,000,000
Noninterest income	2,515,000,000	$ 2,478,000,000	$ 2,071,000,000
Contract assets	0
Contract liabilities	0
Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	427,000,000
Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	260,000,000
Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	362,000,000
Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	263,000,000
Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	18,000,000
Noninterest income	929,000,000
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Noninterest income	2,465,000,000	2,499,000,000	2,089,000,000
Operating Segments [Member] | Key Community Bank [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	846,000,000
Noninterest income	1,098,000,000	1,143,000,000	906,000,000
Operating Segments [Member] | Key Community Bank [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	358,000,000
Operating Segments [Member] | Key Community Bank [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5,000,000
Operating Segments [Member] | Key Community Bank [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	310,000,000
Operating Segments [Member] | Key Community Bank [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	155,000,000
Operating Segments [Member] | Key Community Bank [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	18,000,000
Operating Segments [Member] | Key Corporate Bank [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	484,000,000
Noninterest income	1,161,000,000	1,148,000,000	1,013,000,000
Operating Segments [Member] | Key Corporate Bank [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	69,000,000
Operating Segments [Member] | Key Corporate Bank [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	255,000,000
Operating Segments [Member] | Key Corporate Bank [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	52,000,000
Operating Segments [Member] | Key Corporate Bank [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	108,000,000
Operating Segments [Member] | Key Corporate Bank [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0
Operating Segments [Member] | Other Segments [Member]
Disaggregation of Revenue [Line Items]
Noninterest income	206,000,000	208,000,000	170,000,000
Segment Reconciling Items [Member]
Disaggregation of Revenue [Line Items]
Noninterest income	$ 50,000,000	$ (21,000,000)	$ (18,000,000)