Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 303	$ 542	$ 902	$ 1,523
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $210, $510, $160, and $1,381	(668)	(1,616)	(509)	(4,380)
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $(22), $150, $(50), and $409	72	(477)	161	(1,298)
Net pension and postretirement benefit costs, net of income taxes of $0, $0, $(1), and $(2)	1	2	4	7
Total other comprehensive income (loss), net of tax	(595)	(2,091)	(344)	(5,671)
Comprehensive income (loss)	(292)	(1,549)	558	(4,148)
Less: Comprehensive income attributable to noncontrolling interests	0	0	0	0
Comprehensive income (loss) attributable to Key	$ (292)	$ (1,549)	$ 558	$ (4,148)