Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 1,866	$ 1,298	$ 790
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of ($19), $13, and (76)	(62)	(126)	(127)
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $11, ($19), and ($19)	36	(72)	(34)
Foreign currency translation adjustments, net of income taxes of $11, $9, and ($1)	(23)	12	(1)
Net pension and postretirement benefit costs, net of income taxes of $3, $80, and $19	10	(52)	26
Total other comprehensive income (loss), net of tax	(39)	(238)	(136)
Comprehensive income (loss)	1,827	1,060	654
Less: Comprehensive income attributable to noncontrolling interests	0	2	(1)
Comprehensive income (loss) attributable to Key	$ 1,827	$ 1,058	$ 655