Earnings per share - Summary of Earnings Per Share (Parenthetical) (Detail) (USD $)	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Class A Stock [Member]
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Number of antidilutive warrants, options and restricted shares, for the year	1,057,573	1,058,048
CIBC World Markets Corp. [Member] \| January 2008 Acquisition [Member]
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Period for exercise of warrant	5 years
Acquisition date of CIBC World Markets Corp.	Jan. 14, 2008
Warrants expire date	2013-04-13
CIBC World Markets Corp. [Member] \| Class A Stock [Member] \| January 2008 Acquisition [Member]
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Company issued a warrant to purchase share of Class A Stock	1,000,000
Class A Stock exercisable	48.62

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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-Publisher FASB

-Name Emerging Issues Task Force (EITF)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Diluted Earnings Per Share

-URI http://asc.fasb.org/extlink&oid=6510752

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=6508534

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=6505113

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 128

-Paragraph 171

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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