Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)		3 Months Ended			12 Months Ended
	Aug. 01, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 03, 2014
Goodwill And Other Intangible Asset [Line Items]
Acquisition of First Niagara					$ 1,386,000,000
Expected deductible goodwill for tax purpose					647,000,000
Intangible asset amortization expense					55,000,000	$ 36,000,000	$ 39,000,000
Estimated amortization expense 2017					84,000,000
Estimated amortization expense 2018					70,000,000
Estimated amortization expense 2019					57,000,000
Estimated amortization expense 2020					46,000,000
Estimated amortization expense 2021					$ 39,000,000
Pacific Crest Securities [Member]
Goodwill And Other Intangible Asset [Line Items]
Acquisition of First Niagara				$ 78,000,000
Allocation of acquired goodwill			$ 72,000,000
Increase in goodwill		$ 3,000,000
Intangible assets								$ 13,000,000
Useful life of intangible assets					5 years
First Niagara Bank, N.A. [Member]
Goodwill And Other Intangible Asset [Line Items]
Acquisition of First Niagara	$ 1,386,000,000
Intangible assets acquired	$ 385,000,000
Key Community Bank Unit [Member]
Goodwill And Other Intangible Asset [Line Items]
Estimated fair value of the Community Bank unit greater than certain percent of carrying value						52.00%
Accumulated impairment losses					$ 0	$ 0	0
Key Corporate Bank [Member]
Goodwill And Other Intangible Asset [Line Items]
Estimated fair value of the Community Bank unit greater than certain percent of carrying value						27.00%
Acquisition of First Niagara					277,000,000
Accumulated impairment losses					$ 0	$ 0	$ 0
Consumer Credit Card [Member] | First Niagara Bank, N.A. [Member]
Goodwill And Other Intangible Asset [Line Items]
Useful life of intangible assets					9 years
Commercial Credit Card [Member] | First Niagara Bank, N.A. [Member]
Goodwill And Other Intangible Asset [Line Items]
Useful life of intangible assets					6 years
Core Deposits [Member] | First Niagara Bank, N.A. [Member]
Goodwill And Other Intangible Asset [Line Items]
Useful life of intangible assets					10 years
Intangible assets acquired					$ 356,000,000