Acquisitions and Discontinued Operations - Combined Assets and Liabilities of Discontinued Operations (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011		Dec. 31, 2010
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	$ 974		$ 694		$ 828		$ 278
Loans, net of unearned income of ($2), ($2) and $1	51,419		49,575		48,195
Less: Allowance for loan and lease losses	888		1,004		1,131
Net loans	50,531		48,571		47,064
Trust accrued income and other assets at fair value	2,871	[1]	3,077	[1]	2,946	[1]
Total assets	86,950		88,785		89,262
Trust accrued expense and other liabilities at fair value	1,238		1,763		1,556
Total liabilities	76,671		78,863		79,344
Segment, Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	21		31		31
Loans, net of unearned income of ($2), ($2) and $1	2,744		3,010		3,081
Less: Allowance for loan and lease losses	65		104		115
Net loans	5,263		5,708		5,870
Accrued income and other assets	68		87		101
Total assets	5,381		5,860		6,033
Total liabilities	2,335		2,550		2,651
Segment, Discontinued Operations [Member] | Trust [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust loans accounted for at fair value	2,513		2,726		2,829
Trust accrued income and other assets at fair value	29		34		31
Trust accrued expense and other liabilities at fair value	25		28		28
Trust securities at fair value	2,310		2,522		2,623
Segment, Discontinued Operations [Member] | Portfolio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust loans accounted for at fair value	$ 71		$ 76		$ 75

[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.