Business Combination - Schedule of Purchase Price Allocation (Details) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
	Aug. 01, 2016	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
LIABILITIES
Goodwill			$ 1,400	$ 1,386
First Niagara Bank, N.A. [Member]
Consideration paid:
KeyCorp common stock issued	$ 2,831
Cash payments to First Niagara stockholders	811
Exchange of First Niagara preferred stock for KeyCorp preferred stock	350
Total consideration paid	3,992
ASSETS
Cash and due from banks and short-term investments	620
Investment securities	9,012
Other investments	297
Loans	23,590	$ 23,590
Premises and equipment	245	1,467
Other intangible assets	385
Accrued income and other assets	1,467
Total assets	35,616
LIABILITIES
Deposits	28,994
Bank notes and other short-term borrowings	2,698
Accrued expense and other liabilities	490
Long-term debt	846
Total liabilities	33,028
Net identifiable assets acquired	2,588
Goodwill	$ 1,404			$ 1,386
Business Combination, Provisional Information [Abstract]
Loans		(55)
Premises and equipment, increase (decrease)		18
Unfunded lending-related commitments		65
Unfunded lending-related commitments, increase (decrease)		(2)
Deferred compensation		41
Deferred compensation, increase (decrease)		2
Scenario, Previously Reported [Member] | First Niagara Bank, N.A. [Member]
ASSETS
Loans		23,645
Premises and equipment		1,449
Business Combination, Provisional Information [Abstract]
Unfunded lending-related commitments		67
Deferred compensation		$ 39
Interest Income [Member] | First Niagara Bank, N.A. [Member]
Business Combination, Provisional Information [Abstract]
Loans			42
Provision for Credit Losses [Member] | First Niagara Bank, N.A. [Member]
Business Combination, Provisional Information [Abstract]
Loans			1
Other Noninterest Income [Member] | First Niagara Bank, N.A. [Member]
Business Combination, Provisional Information [Abstract]
Loans			(3)
Unfunded lending-related commitments, increase (decrease)			$ (4)