Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 334		$ 981
Noninterest income	609	$ 592	1,870	$ 1,822
Contract assets	0		0
Contract liabilities	0		0
Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	106		318
Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	70		185
Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	85		265
Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	68		197
Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5		16
Noninterest income	235		691
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Noninterest income	606	593	1,811	1,837
Operating Segments [Member] | Community Banking [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	208		626
Noninterest income	268	272	820	856
Operating Segments [Member] | Community Banking [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	90		268
Operating Segments [Member] | Community Banking [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1		2
Operating Segments [Member] | Community Banking [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	72		226
Operating Segments [Member] | Community Banking [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	40		115
Operating Segments [Member] | Community Banking [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5		15
Operating Segments [Member] | Key Corporate Bank [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	126		355
Noninterest income	301	$ 269	852	$ 827
Operating Segments [Member] | Key Corporate Bank [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	16		50
Operating Segments [Member] | Key Corporate Bank [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	69		183
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		39
Operating Segments [Member] | Key Corporate Bank [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	28		82
Operating Segments [Member] | Key Corporate Bank [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 0		$ 1