Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 416	$ 482	$ 1,248	$ 1,382
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of ($29), ($22), ($176), and ($89)	93	(72)	568	(288)
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of ($21), ($5), ($120) and ($29)	68	(12)	386	(92)
Foreign currency translation adjustments, net of income taxes of $0, $0, ($1) and $3	0	1	4	(15)
Net pension and postretirement benefit costs, net of income taxes of $0, $1, ($6) and $3	0	2	19	8
Total other comprehensive income (loss), net of tax	161	(81)	977	(387)
Comprehensive income (loss)	577	401	2,225	995
Less: Comprehensive income attributable to noncontrolling interests	0	0	0	0
Comprehensive income (loss) attributable to Key	$ 577	$ 401	$ 2,225	$ 995