Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 482	$ 364	$ 1,382	$ 1,101
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of ($22), $0, ($89), and $24	(72)	0	(288)	40
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of ($5), ($6), ($29), and ($16)	(12)	(10)	(92)	(27)
Foreign currency translation adjustments, net of income taxes of $0, $4, $3, and $8	1	7	(15)	14
Net pension and postretirement benefit costs, net of income taxes of $1, $1, $3, and $4	2	3	8	8
Total other comprehensive income (loss), net of tax	(81)	0	(387)	35
Comprehensive income (loss)	401	364	995	1,136
Less: Comprehensive income attributable to noncontrolling interests	0	0	0	1
Comprehensive income (loss) attributable to Key	$ 401	$ 364	$ 995	$ 1,135