Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 219	$ 312
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $47 and $(181)	(151)	575
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $(20) and $(77)	65	245
Net pension and postretirement benefit costs, net of income taxes of $0 and $0	1	1
Total other comprehensive income (loss), net of tax	(85)	821
Comprehensive income (loss) attributable to Key	$ 134	$ 1,133