Fair Value Measurements - Additional Information (Detail) (USD $)	6 Months Ended	9 Months Ended
	Jun. 30, 2011 Entity	Sep. 30, 2012		Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of independent entities	2
Unfunded commitments		$ 103,000,000
Investment liquidation period		1 year
Investment liquidation period		10 years
Period for evaluation of fair value of property		90 days
Nonexit price methodology for valuing loan portfolio					0.40%
Loan, net of allowance		50,531,000,000			48,571,000,000	47,064,000,000
Loan in portfolio		71,000,000	[1]
Loans held for sale	381,000,000	628,000,000		656,000,000	728,000,000	479,000,000
Average maturity of short term financial instrument		Less than six months
Segment, Discontinued Operations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan, net of allowance		5,263,000,000			5,708,000,000	5,870,000,000
Segment, Discontinued Operations [Member] | Education Lending [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan, net of allowance		5,263,000,000			5,708,000,000	5,869,000,000
Loan in portfolio		71,000,000			76,000,000
Residential Real Estate Mortgage Loans [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan, net of allowance		2,100,000,000			1,900,000,000
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial loan held-for-sale		628,000,000			728,000,000
Fair Value [Member] | Segment, Discontinued Operations [Member] | Education Lending [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan, net of allowance		2,300,000,000			2,500,000,000
Loans in the trusts		2,500,000,000			2,700,000,000
Securities issued by the education lending securitization trust		2,300,000,000			2,500,000,000
Carrying Value Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial loan held-for-sale		628,000,000			728,000,000
Carrying Value Amount [Member] | Segment, Discontinued Operations [Member] | Education Lending [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan, net of allowance		2,700,000,000			2,900,000,000
Loans held for sale		0			0
Principal direct investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total other investments		455,000,000
Level 3 [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial loan held-for-sale		628,000,000
Level 3 [Member] | Commercial loans [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial loan held-for-sale		$ 10,000,000			$ 15,000,000

[1] The assets of the VIEs can only be used by the particular VIE and there is no recourse to Key with respect to the liabilities of the consolidated LIHTC or education loan securitization trust VIEs.