Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 274	$ 287	$ 493	$ 599
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $(18), $131, $29, and $(50)	59	(416)	(92)	159
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $(35), $49, $(55), and $(28)	109	(156)	174	89
Net pension and postretirement benefit costs, net of income taxes of $(1), $(1), $(1), and $(1)	2	2	3	3
Total other comprehensive income (loss), net of tax	170	(570)	85	251
Comprehensive income (loss) attributable to Key	$ 444	$ (283)	$ 578	$ 850