Acquisitions and Discontinued Operations - Components of Income (Loss) from Discontinued Operations, Net of Taxes, of Austin Capital Management (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Noninterest income	$ 1,967	$ 1,808	$ 1,954
Noninterest expense	2,907	2,790	3,034
Income taxes	239	369	186
Income (loss) from discontinued operations, net of taxes	9	(44)	(23)
Segment, discontinued operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Noninterest income	49	54	61
Noninterest expense	46	41	54
Income (loss) before income taxes	15	(70)	(37)
Income taxes	6	(26)	(14)
Income (loss) from discontinued operations, net of taxes	9	(44)	(23)
Segment, discontinued operations [Member] | Austin Capital Management [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Noninterest income		1	5
Noninterest expense	10	2	6
Income (loss) before income taxes	(10)	(1)	(1)
Income taxes	(3)
Income (loss) from discontinued operations, net of taxes	$ (7)	$ (1)	$ (1)