Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 287	$ 533	$ 599	$ 981
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $131, $309, $(50), and $871	(416)	(980)	159	(2,764)
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $49, $82, $(28), and $259	(156)	(260)	89	(821)
Net pension and postretirement benefit costs, net of income taxes of $(1), $(1), $(1), and $(2)	2	3	3	5
Total other comprehensive income (loss), net of tax	(570)	(1,237)	251	(3,580)
Comprehensive income (loss)	(283)	(704)	850	(2,599)
Less: Comprehensive income attributable to noncontrolling interests	0	0	0	0
Comprehensive income (loss) attributable to Key	$ (283)	$ (704)	$ 850	$ (2,599)