Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 489	$ (410)	$ 1,319	$ 83
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $(97), $(524), $(281) and $(495)	301	1,663	877	1,571
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $(15), $(62), $(120) and $(117)	50	199	376	373
Net pension and postretirement benefit costs, net of income taxes of $0, $0, $(21) and $(1)	1	1	68	4
Total other comprehensive income (loss), net of tax	352	1,863	1,321	1,948
Comprehensive income (loss) attributable to Key	$ 841	$ 1,453	$ 2,640	$ 2,031