Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
ASSETS
Loans, net of allowance	$ 154	$ 157	$ 74
Derivative assets	609	693	830
LIABILITIES
Derivative liabilities	524	584	754
Carrying Amount [Member]
ASSETS
Cash and short-term investments	3,702	4,525	4,021
Trading account assets	701	605	614
Securities available for sale	13,496	12,094	14,633
Held-to-maturity securities	3,721	3,931	3,019
Other investments	1,059	1,064	1,188
Loans, net of allowance	51,681	51,934	48,282
Loans held for sale	434	599	511
Mortgage servicing assets	201	204	183
Derivative assets	609	693	830
LIABILITIES
Deposits with no stated maturity	56,810	58,132	50,805
Time deposits	7,844	7,861	10,689
Short-term borrowings	2,328	1,896	2,170
Long-term debt	7,785	6,847	8,898
Derivative liabilities	524	584	754
Fair Value [Member]
ASSETS
Cash and short-term investments	3,702	4,525	4,021
Trading account assets	701	605	614
Securities available for sale	13,496	12,094	14,633
Held-to-maturity securities	3,779	3,992	3,052
Other investments	1,059	1,064	1,188
Loans, net of allowance	50,926	51,046	47,348
Loans held for sale	434	599	511
Mortgage servicing assets	244	238	226
Derivative assets	609	693	830
LIABILITIES
Deposits with no stated maturity	56,810	58,132	50,805
Time deposits	7,982	8,020	10,953
Short-term borrowings	2,328	1,896	2,170
Long-term debt	8,263	7,392	9,131
Derivative liabilities	524	584	754
Fair Value [Member] \| Level 1 [Member]
ASSETS
Cash and short-term investments	3,391	4,254	3,729
Trading account assets	5	2	11
Securities available for sale	43	43	12
Held-to-maturity securities
Other investments			18
Loans, net of allowance
Loans held for sale
Mortgage servicing assets
Derivative assets	57	54	73
LIABILITIES
Deposits with no stated maturity
Time deposits	522	408	857
Short-term borrowings	3		6
Long-term debt	2,781	2,807	3,885
Derivative liabilities	54	54	64
Fair Value [Member] \| Level 2 [Member]
ASSETS
Cash and short-term investments	311	271	292
Trading account assets	693	600	602
Securities available for sale	13,453	12,051	14,621
Held-to-maturity securities	3,779	3,992	3,052
Other investments	394	396	402
Loans, net of allowance
Loans held for sale
Mortgage servicing assets
Derivative assets	1,630	1,883	2,010
LIABILITIES
Deposits with no stated maturity	56,810	58,132	50,805
Time deposits	7,460	7,612	10,096
Short-term borrowings	2,325	1,896	2,164
Long-term debt	5,482	4,585	5,246
Derivative liabilities	1,164	1,331	1,535
Fair Value [Member] \| Level 3 [Member]
ASSETS
Cash and short-term investments
Trading account assets	3	3	1
Securities available for sale
Held-to-maturity securities
Other investments	665	668	768
Loans, net of allowance	50,926	51,046	47,348
Loans held for sale	434	599	511
Mortgage servicing assets	244	238	226
Derivative assets	37	26	42
LIABILITIES
Deposits with no stated maturity
Time deposits
Short-term borrowings
Long-term debt
Derivative liabilities	2	2	2
Fair Value [Member] \| Netting Adjustment [Member]
ASSETS
Cash and short-term investments
Trading account assets
Securities available for sale
Held-to-maturity securities
Other investments
Loans, net of allowance
Loans held for sale
Mortgage servicing assets
Derivative assets	(1,115)	(1,270)	(1,295)
LIABILITIES
Deposits with no stated maturity
Time deposits
Short-term borrowings
Long-term debt
Derivative liabilities	$ (696)	$ (803)	$ (847)

Cash and short term investments fair value.

No definition available.

Name:	key_CashAndShortTermInvestmentsFairValue
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Deposits with no stated maturity fair value.

+ References

No definition available.

+ Details

Name:	key_DepositsWithNoStatedMaturityFairValue
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Mortgage servicing assets fair value.

+ References

No definition available.

+ Details

Name:	key_MortgageServicingAssetsFairValue
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Other short term investments fair value.

+ References

No definition available.

+ Details

Name:	key_OtherShortTermInvestmentsFairValue
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Short term borrowing fair value.

+ References

No definition available.

+ Details

Name:	key_ShortTermBorrowingFairValue
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_AssetsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

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

- Definition

+ References

No definition available.

+ Details

Name:	us-gaap_DepositsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FIN39-1

-Paragraph 10A, 10B

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 4, 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611

+ Details

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

- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 33

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 7-11

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Details

Name:	us-gaap_LiabilitiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

+ References

No definition available.

+ Details

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

- Definition

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Glossary Loan

-URI http://asc.fasb.org/extlink&oid=6517073

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 114

-Paragraph 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 4

-Article 9

+ Details

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