Acquisitions and dispositions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		1 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011 Shareowner Services	Jun. 30, 2011 Talon Asset Management Year	Nov. 30, 2011 Penson Financial Services Australia Pty Ltd Year
Business Acquisition [Line Items]
Contingent payments	$ 3	$ 6
Potential obligation to pay additional consideration, lower limit	2
Potential obligation to pay additional consideration, upper limit	36
Potential obligation to pay additional consideration, period of measurement (in years)	3 years
Business acquisition				11	33
Potential contingent additional consideration recorded as goodwill				5
Goodwill related to acquisition, tax deductible				10	10
Intangible assets, useful life (in years)				20	9
Intangible assets - customer relationships				6	6
Business disposition, sales price			550
Business disposition, pre-tax gain			$ 98