Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Beginning Balance	$ 2,538	$ 2,446
KIBS divestiture	(22)
Ending Balance	2,516	2,538
Key Community Bank [Member]
Goodwill [Roll Forward]
Beginning Balance	2,124	2,088
KIBS divestiture	(22)
Ending Balance	2,102	2,124
Key Corporate Bank [Member]
Goodwill [Roll Forward]
Beginning Balance	414	358
KIBS divestiture	0
Ending Balance	$ 414	414
First Niagara Bank, N.A. [Member]
Goodwill [Roll Forward]
Fair value measurement adjustments - First Niagara acquisition		18
First Niagara Bank, N.A. [Member] \| Key Community Bank [Member]
Goodwill [Roll Forward]
Fair value measurement adjustments - First Niagara acquisition		15
First Niagara Bank, N.A. [Member] \| Key Corporate Bank [Member]
Goodwill [Roll Forward]
Fair value measurement adjustments - First Niagara acquisition		3
Key Merchant Services, LLC [Member]
Goodwill [Roll Forward]
Fair value measurement adjustments - First Niagara acquisition		4
Additional ownership interest in Key Merchant Services		4
Key Merchant Services, LLC [Member] \| Key Community Bank [Member]
Goodwill [Roll Forward]
Additional ownership interest in Key Merchant Services		4
Key Merchant Services, LLC [Member] \| Key Corporate Bank [Member]
Goodwill [Roll Forward]
Additional ownership interest in Key Merchant Services		0
HelloWallet Holdings, LLC [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period		17
HelloWallet Holdings, LLC [Member] \| Key Community Bank [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period		17
HelloWallet Holdings, LLC [Member] \| Key Corporate Bank [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period		0
Cain Brothers & Company, LLC. [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period		53
Cain Brothers & Company, LLC. [Member] \| Key Community Bank [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period		0
Cain Brothers & Company, LLC. [Member] \| Key Corporate Bank [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period		$ 53

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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