Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 645	$ 428	$ 1,996	$ 761
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $56, $(3), $195, and $163	(174)	(8)	(617)	527
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $9, $(19), $57, and $93	(29)	(64)	(180)	299
Net pension and postretirement benefit costs, net of income taxes of $(3), $1, $(5), and $5	8	3	14	15
Total other comprehensive income (loss), net of tax	(195)	(69)	(783)	841
Comprehensive income (loss)	450	359	1,213	1,602
Less: Comprehensive income attributable to noncontrolling interests	0	0	0	0
Comprehensive income (loss) attributable to Key	$ 450	$ 359	$ 1,213	$ 1,602