Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 729	$ 187	$ 1,351	$ 333
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $(59), $40, $139, and $166	185	130	(443)	535
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $45, $(5), $48, and $112	(141)	(14)	(151)	363
Foreign currency translation adjustments, net of income taxes of $0, $0, $0, and $0	0	0	0	0
Net pension and postretirement benefit costs, net of income taxes of $(1), $2, $(2), and $4	3	6	6	12
Total other comprehensive income (loss), net of tax	47	122	(588)	910
Comprehensive income (loss)	776	309	763	1,243
Less: Comprehensive income attributable to noncontrolling interests	0	0		0
Comprehensive income (loss) attributable to Key	$ 776	$ 309	$ 763	$ 1,243