Business Combination (Details)		12 Months Ended
	Aug. 01, 2016 USD ($) Branch $ / shares shares	Dec. 31, 2016 USD ($)	Sep. 09, 2016 Branch	Jul. 29, 2016 $ / shares
Business Acquisition [Line Items]
Number of branch locations acquired | Branch	390
Number of branches sold | Branch			18
Goodwill		$ 1,386,000,000
First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Total consideration	$ 3,992,000,000
Maximum period for change in fair value estimate		1 year
Intangible assets acquired	385,000,000
Goodwill	$ 1,386,000,000
Merger and acquisition integration costs		$ 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
Purchase Credit Impaired Loans [Member] | First Niagara Bank, N.A. [Member]
Business Acquisition [Line Items]
Contractually required payments receivable	22,600,000,000
Fair value of acquired receivables	22,200,000,000
Estimated cash flows not expected to be collected	$ 399,000,000
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		$ 1,109,000,000
Key Corporate Bank [Member]
Business Acquisition [Line Items]
Goodwill		$ 277,000,000