Business Combination - Narrative (Details)		12 Months Ended
	Aug. 01, 2016 USD ($) Branch $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jul. 29, 2016 $ / shares
Business Acquisition [Line Items]
Goodwill		$ 1,400,000,000	$ 1,386,000,000
First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Total consideration	$ 3,992,000,000
Number of branch locations acquired | Branch	390
Intangible assets acquired	$ 385,000,000
Goodwill	$ 1,404,000,000		1,386,000,000
Merger and acquisition integration costs		1,000,000	44,000,000
Common Stock [Member] | First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Shares available for issuance upon conversion of acquiree stock | shares	0.680
Cash paid upon conversion of acquiree stock	$ 2.30
Shares received by acquiree (in dollars per share) | $ / shares	$ 10.26
Closing price of acquiree stock (in dollars per share) | $ / shares				$ 11.70
Number of shares received by acquiree | shares	240,000,000
Fair value of acquiree options to purchase common stock	$ 26,000,000
Core Deposits [Member] | First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Intangible assets acquired		$ 356,000,000
Useful life of intangible assets		10 years
Commercial Credit Card [Member] | First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Useful life of intangible assets		6 years
Consumer Credit Card [Member] | First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Useful life of intangible assets		9 years
Key Community Bank [Member]
Business Acquisition [Line Items]
Goodwill		$ 1,100,000,000	1,109,000,000
Key Community Bank [Member] | First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Goodwill			1,109,000,000
Key Corporate Bank [Member]
Business Acquisition [Line Items]
Goodwill		$ 280,000,000	277,000,000
Key Corporate Bank [Member] | First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Goodwill			$ 277,000,000