Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 342		$ 646
Noninterest income	660	$ 653	1,261	$ 1,230
Contract assets	0		0
Contract liabilities	0		0
Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	109		212
Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	68		115
Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	91		179
Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	69		130
Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5		10
Noninterest income	204		457
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Noninterest income	596	661	1,204	1,244
Operating Segments [Member] | Community Banking [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	215		418
Noninterest income	281	322	551	583
Operating Segments [Member] | Community Banking [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	91		179
Operating Segments [Member] | Community Banking [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1		1
Operating Segments [Member] | Community Banking [Member] | Service Charges On Deposit Accounts [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	78		153
Operating Segments [Member] | Community Banking [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	40		75
Operating Segments [Member] | Community Banking [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5		10
Operating Segments [Member] | Key Corporate Bank [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	127		228
Noninterest income	265	$ 285	552	$ 559
Operating Segments [Member] | Key Corporate Bank [Member] | Trust And Investment Services [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	18		33
Operating Segments [Member] | Key Corporate Bank [Member] | Investment Banking And Debt Placement [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	67		114
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		26
Operating Segments [Member] | Key Corporate Bank [Member] | Cards And Payments [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	29		55
Operating Segments [Member] | Key Corporate Bank [Member] | Other Noninterest Income [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 0		$ 0