Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 1,343	$ 1,717	$ 1,866
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $(143), $(151), and $(19)	452	488	(62)
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $(72), $(93), and $11	226	300	36
Foreign currency translation adjustments, net of income taxes of $0, $(4), and $11	0	14	(23)
Net pension and postretirement benefit costs, net of income taxes of $(9), $(13), and $3	34	42	10
Total other comprehensive income (loss), net of tax	712	844	(39)
Comprehensive income (loss)	2,055	2,561	1,827
Less: Comprehensive income attributable to noncontrolling interests	0	0	0
Comprehensive income (loss) attributable to Key	$ 2,055	$ 2,561	$ 1,827