Business Combination - Schedule of Purchase Price Calculation and Identifiable Assets Purchased and Liabilities Assumed (Details) - USD ($) $ in Millions		9 Months Ended
	Aug. 01, 2016	Sep. 30, 2016
LIABILITIES
Goodwill		$ 1,420
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	9,019
Other investments	297
Loans	23,504
Premises and equipment	276
Other intangible assets	388
Accrued income and other assets	1,449
Total assets	35,553
LIABILITIES
Deposits	28,993
Bank notes and other short-term borrowings	2,698
Accrued expense and other liabilities	444
Long-term debt	846
Total liabilities	32,981
Net identifiable assets acquired	2,572
Goodwill	$ 1,420