Business Combination - Schedule of Changes in Goodwill by Reporting Unit (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning Balance	$ 2,446	$ 2,446	$ 1,060
Acquisition of First Niagara		1,400	1,386
Tax adjustment on previous fair value measurements	(19)	18
Loan adjustment on previous fair value measurements	37
Ending Balance	2,464	2,538	2,446
Key Community Bank [Member]
Goodwill [Roll Forward]
Beginning Balance	2,088	2,088	979
Acquisition of First Niagara		1,100	1,109
Tax adjustment on previous fair value measurements	(15)	15
Loan adjustment on previous fair value measurements	30
Ending Balance	2,103	2,124	2,088
Key Corporate Bank [Member]
Goodwill [Roll Forward]
Beginning Balance	358	358	81
Acquisition of First Niagara		280	277
Tax adjustment on previous fair value measurements	(4)	3
Loan adjustment on previous fair value measurements	7
Ending Balance	$ 361	$ 414	$ 358

Goodwill, Purchase Accounting Adjustments, Loan Adjustment

No definition available.

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- Definition

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.

+ References

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

+ Details

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- Definition

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.

+ References

+ Details

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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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- Definition

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

+ References

No definition available.

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