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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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

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Amount of increase (decrease), classified as other, of an asset representing the future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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- Definition

Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
	Aug. 01, 2016	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning Balance		$ 2,446	$ 2,446	$ 1,060
Fair value measurement adjustments - First Niagara acquisition		(19)	18
Additional ownership interest in Key Merchant Services			4
Goodwill, acquired during period			1,400	1,386
Ending Balance		2,464	2,538	2,446
Key Community Bank [Member]
Goodwill [Roll Forward]
Beginning Balance		2,088	2,088	979
Fair value measurement adjustments - First Niagara acquisition		(15)	15
Additional ownership interest in Key Merchant Services			4
Goodwill, acquired during period			1,100	1,109
Ending Balance		2,103	2,124	2,088
Key Corporate Bank [Member]
Goodwill [Roll Forward]
Beginning Balance		358	358	81
Fair value measurement adjustments - First Niagara acquisition		(4)	3
Additional ownership interest in Key Merchant Services			0
Goodwill, acquired during period			280	277
Ending Balance		$ 361	414	358
First Niagara Bank, N.A. [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period	$ 1,404			1,386
First Niagara Bank, N.A. [Member] \| Key Community Bank [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period				1,109
First Niagara Bank, N.A. [Member] \| Key Corporate Bank [Member]
Goodwill [Roll Forward]
Goodwill, acquired during period				$ 277
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