Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 1,717	$ 1,866	$ 1,298
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $(151), ($19), and $13	488	(62)	(126)
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $(93), $11, and ($19)	300	36
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $(93), $11, and ($19)			(72)
Foreign currency translation adjustments, net of income taxes of $(4), $11, and $9	14	(23)	12
Net pension and postretirement benefit costs, net of income taxes of $(13), $3, and $80	42	10	(52)
Total other comprehensive income (loss), net of tax	844	(39)	(238)
Comprehensive income (loss)	2,561	1,827	1,060
Less: Comprehensive income attributable to noncontrolling interests	0	0	2
Comprehensive income (loss) attributable to Key	$ 2,561	$ 1,827	$ 1,058