Derivatives and Hedging Activities - Effect of Fair Value and Cash Flow Hedges on Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense — Long-term debt		$ (114)	$ (108)	$ (354)	$ (302)
Interest income — Loans		1,073	1,025	3,221	2,965
Deposits		(227)	(140)	(652)	(343)
Other income	[1]	27	23	54	143
Interest Rate [Member] | Interest Expense, Long-Term Debt [Member] | Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Recognized on hedged items		(75)	41	(320)	134
Recognized on derivatives designated as hedging instruments		72	(46)	296	(140)
Net income (expense) recognized on fair value hedges		(3)	(5)	(24)	(6)
Interest Rate [Member] | Interest Expense, Long-Term Debt [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Realized gains (losses) (pre-tax) reclassified from AOCI into net income		(1)	(2)	(2)	(2)
Net income (expense) recognized on cash flow hedges		(1)	(2)	(2)	(2)
Interest Rate [Member] | Interest Income [Member] | Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Recognized on hedged items		0	0	0	0
Recognized on derivatives designated as hedging instruments		0	0	0	0
Net income (expense) recognized on fair value hedges		0	0	0	0
Interest Rate [Member] | Interest Income [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Realized gains (losses) (pre-tax) reclassified from AOCI into net income		(8)	(15)	(51)	(42)
Net income (expense) recognized on cash flow hedges		(8)	(15)	(51)	(42)
Interest Rate [Member] | Interest Expense, Deposits [Member] | Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Recognized on hedged items		0	0	(1)	1
Recognized on derivatives designated as hedging instruments		0	0	0	0
Net income (expense) recognized on fair value hedges		0	0	(1)	1
Interest Rate [Member] | Interest Expense, Deposits [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Realized gains (losses) (pre-tax) reclassified from AOCI into net income		0	0	0	0
Net income (expense) recognized on cash flow hedges		0	0	0	0
Interest Rate [Member] | Other Income [Member] | Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Recognized on hedged items		0	0	0	0
Recognized on derivatives designated as hedging instruments		0	0	0	0
Net income (expense) recognized on fair value hedges		0	0	0	0
Interest Rate [Member] | Other Income [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Realized gains (losses) (pre-tax) reclassified from AOCI into net income		0	0	0	0
Net income (expense) recognized on cash flow hedges		$ 0	$ 0	$ 0	$ 0

[1] For the three and nine months ended September 30, 2019, net securities gains (losses) totaled $15 million. For the three and nine months ended September 30, 2018, net securities gains (losses) totaled less than $1 million. For the three and nine months ended September 30, 2019, and September 30, 2018, Key did not have any impairment losses related to securities.