Acquisitions and Discontinued Operations - Combined Assets and Liabilities of Discontinued Operations (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012		Dec. 31, 2011
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	$ 748		$ 584		$ 973		$ 693
Trust loans accounted for at fair value	148		157		71
Loans, net of unearned income of ($6), ($5), and ($2)	53,597		52,822		51,419
Less: Allowance for loan and lease losses	868		888		888
Net loans	52,729		51,934		50,531
Accrued income and other assets	2,747	[1]	2,774	[1]	2,853	[1]
Total assets	90,708		89,236		86,950
Trust accrued expense and other liabilities at fair value	1,375		1,387		1,205
Total liabilities	80,472		78,927		76,671
Segment, Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	20		23		22
Loans, net of unearned income of ($6), ($5), and ($2)	2,455		2,675		2,744
Less: Allowance for loan and lease losses	38		55		65
Net loans	4,700		5,146		5,263
Accrued income and other assets	95		87		86
Total assets	4,838		5,282		5,400
Accrued expense and other liabilities			39		33
Total liabilities	2,037		2,220		2,368
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	2,135		2,369		2,513
Trust accrued income and other assets at fair value	23		26		29
Trust accrued expense and other liabilities at fair value	21		22		25
Trust securities at fair value	2,016		2,159		2,310
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	$ 148		$ 157		$ 71

[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.