Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 407	$ 418
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $56 and ($47)	184	(150)
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of $31 and ($20)	99	(63)
Foreign currency translation adjustments, net of income taxes of $0 and $0	3	(2)
Net pension and postretirement benefit costs, net of income taxes of $1 and $1	2	3
Total other comprehensive income (loss), net of tax	288	(212)
Comprehensive income (loss)	695	206
Less: Comprehensive income attributable to noncontrolling interests	0	0
Comprehensive income (loss) attributable to Key	$ 695	$ 206