Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Beginning Balance	$ 1,060	$ 1,057
Acquisition of First Niagara	1,386
Ending Balance	2,446	1,060
Key Community Bank [Member]
Goodwill [Roll Forward]
Beginning Balance	979	979
Tax adjustment resulting from Pacific Crest Securities acquisition		0
Acquisition of First Niagara	1,109
Ending Balance	2,088	979
Key Corporate Bank [Member]
Goodwill [Roll Forward]
Beginning Balance	81	78
Acquisition of First Niagara	277
Ending Balance	$ 358	$ 81