Loans and Loans Held for Sale - Summary of Changes in Loans Held for Sale (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Loans Receivables [Abstract]
Balance at beginning of the period	$ 611us-gaap_LoansReceivableHeldForSaleNet	$ 599us-gaap_LoansReceivableHeldForSaleNet
New originations	5,681	5,452
Transfers from (to) held to maturity, net	(3)	52
Loan sales	(5,289)	(5,480)
Loan draws (payments), net	(266)	(12)
Balance at end of period	$ 734us-gaap_LoansReceivableHeldForSaleNet	$ 611us-gaap_LoansReceivableHeldForSaleNet

Net transfers from held to maturity.

No definition available.

Name:	key_LoansReceivableHeldForSaleReconciliationToCashFlowTransfersHeldToMaturity
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	key_LoansReceivablesAbstract
Namespace Prefix:	key_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount classified as loans attributable to disposal group held for sale or disposed of. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph a

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(a))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 35

-Paragraph 49

-URI http://asc.fasb.org/extlink&oid=35742846&loc=d3e3084-111518

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 310

-Section 30

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6490209&loc=d3e48214-111002

+ Details

Name:	us-gaap_LoansReceivableHeldForSaleNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reconcile the changes in loans that will be sold to other entities balances to the amounts presented in the consolidated statement of cash flows. Enter the additions to trade accounts that will be sold to other entities for the period.

+ References

+ Details

Name:	us-gaap_LoansReceivableHeldForSaleReconciliationToCashFlowAdditionsToHeldForSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reconcile the changes in loans that will be sold to other entities balances to the amounts presented in the consolidated statement of cash flows. Enter the deductions to trade accounts that will be sold to other entities for the period.

+ References

+ Details

Name:	us-gaap_LoansReceivableHeldForSaleReconciliationToCashFlowDeductionsFromHeldForSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate cash outflow used to purchase all loans that are held with the intention to resell in the near future.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 21

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3461-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 230

-Section 55

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=34726456&loc=d3e60097-112785

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3367-108585

+ Details

Name:	us-gaap_PaymentsToPurchaseLoansHeldForSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration