Acquisitions and dispositions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended	1 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011 Shareowner Services	Jun. 30, 2011 Talon Asset Management	Nov. 30, 2011 Penson Financial Services Australia Pty Ltd
Business Acquisition [Line Items]
Contingent payments	$ 1	$ 4
Potential obligation to pay additional consideration, lower limit	3	3
Potential obligation to pay additional consideration, upper limit	35	35
Potential obligation to pay additional consideration, period of measurement (in years)		2 years
Business acquisition				11	33
Potential contingent additional consideration recorded as goodwill				5
Goodwill related to acquisition, tax deductible				10
Intangible assets, useful life (in years)				20 years	9 years
Intangible assets - customer relationships				6	6
Goodwill related to acquisition, non-tax deductible					10
Business disposition, sales price			550
Business disposition, pre-tax gain			$ 98