Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 428	$ 416	$ 761	$ 1,248
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of ($3), ($29), $163 and ($176)	(8)	93	527	568
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of ($19), ($21), $93, and ($120)	(64)	68	299	386
Foreign currency translation adjustments, net of income taxes of $0, $0, $0, and ($1)	0		0	4
Net pension and postretirement benefit costs, net of income taxes of $1, $0, $5, and ($6)	3	0	15	19
Total other comprehensive income (loss), net of tax	(69)	161	841	977
Comprehensive income (loss)	359	577	1,602	2,225
Less: Comprehensive income attributable to noncontrolling interests	0	0	0	0
Comprehensive income (loss) attributable to Key	$ 359	$ 577	$ 1,602	$ 2,225