Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 320
Noninterest income	601	$ 577
Contract assets	0
Contract liabilities	0
Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	119
Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	47
Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	89
Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	60
Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5
Noninterest income	252
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Noninterest income	608	584
Operating Segments [Member] | Community Banking [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	218
Noninterest income	285	277
Operating Segments [Member] | Community Banking [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	103
Operating Segments [Member] | Community Banking [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0
Operating Segments [Member] | Community Banking [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	76
Operating Segments [Member] | Community Banking [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	34
Operating Segments [Member] | Community Banking [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5
Operating Segments [Member] | Key Corporate Bank [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	102
Noninterest income	287	$ 274
Operating Segments [Member] | Key Corporate Bank [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	16
Operating Segments [Member] | Key Corporate Bank [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	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	13
Operating Segments [Member] | Key Corporate Bank [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	26
Operating Segments [Member] | Key Corporate Bank [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 0