Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 187	$ 425	$ 333	$ 832
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on securities available for sale, net of income taxes of $40, $91, $166 and $147	130	291		475
Net unrealized gains (losses) on derivative financial instruments, net of income taxes of ($5), $68. $112, and $99	(14)	219		318
Foreign currency translation adjustments, net of income taxes of $0, $1, $0, and $1	0	1		4
Net pension and postretirement benefit costs, net of income taxes of $2, $5, $4, and $6	6	17		19
Total other comprehensive income (loss), net of tax	122	528	910	816
Comprehensive income (loss)	309	953	1,243	1,648
Less: Comprehensive income attributable to noncontrolling interests	0	0		0
Comprehensive income (loss) attributable to Key	$ 309	$ 953	$ 1,243	$ 1,648