Financial instruments - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended	3 Months Ended			3 Months Ended		12 Months Ended			3 Months Ended						3 Months Ended			3 Months Ended
	Mar. 31, 2013 Bank ForwardContracts	Dec. 31, 2012	Mar. 31, 2013 U.S. Treasury Notes Securities [Member] Contract	Mar. 31, 2013 Deferred Compensation Share Based Payments [Member] Corporate equities liabilities [Member]	Dec. 31, 2012 Deferred Compensation Share Based Payments [Member] Corporate equities liabilities [Member]	Mar. 31, 2013 Interest Rate Cap [Member]	Mar. 31, 2013 Fixed income investments [Member] U.S. Treasury Notes Securities [Member]	Dec. 31, 2012 Senior Secured Credit Note [Member]	Mar. 31, 2013 Corporate [Member]	Mar. 31, 2013 Other Than Corporate Customer [Member]	Mar. 31, 2012 Credit Concentration Risk [Member]	Mar. 31, 2013 Credit Concentration Risk [Member]	Mar. 31, 2013 Reverse repurchase agreement [Member]	Dec. 31, 2012 Reverse repurchase agreement [Member]	Mar. 31, 2013 Securities Borrowed Transactions [Member]	Dec. 31, 2012 Securities Borrowed Transactions [Member]	Mar. 31, 2013 Warehouse Facility [Member] Credit Concentration Risk [Member]	Mar. 31, 2013 Lehman Brothers International Europe [Member]	Mar. 31, 2013 Three Major U.S. Broker [Member]	Mar. 31, 2013 Auction Rate Securities [Member]	Mar. 31, 2013 Auction Rate Securities [Member] Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of the corporate equities that are included in securities owned				$ 13,300,000	$ 14,000,000
Company purchased and holds auction rate securities																				78,200,000
Commitment to purchase ARS from clients through 2016																				39,300,000
Commitment to purchase ARS period maximum																					2016
ARS held in proprietary trading account result of the failed auctions																				150,000
Valuation adjustment (unrealized loss) for ARS																				7,900,000
Assets and liabilities transferred from level one and level two																				0
Loan position held in secondary loan trading portfolio	0	0
Fair value of the reverse repurchase agreements
Fair value of the repurchase agreements
Maturity date of interest rate cap contract		December 31, 2012
Senior Secured Credit Note reference rate description								3-Month LIBOR
Interest expense						19,200
Number of foreign exchange option outstanding	0
Forward or delayed delivery of the underlying instrument with settlement	180 days
Number of open short contract			388
Open short contract, period			10 years
Open contracts for Federal Funds futures			1,466
Open contracts for Federal Funds futures with a fair value			71,000				360,000
Bank call loans	204,000,000	128,300,000
Collateralized loans, collateralized by firm									141,500,000
Collateralized loans, collateralized by customer securities										278,200,000
Number of money center banks	2
Customer securities under customer margin loans available to be pledged	1,200,000,000
Amount repledged of customer securities under customer margin loan	154,600,000												14,700,000	14,300,000	3,900,000,000	1,100,000,000
Customer securities deposited to secure obligations and margin requirements under option contracts	336,200,000
Outstanding letters of credit	0
Securities received as collateral under securities borrowed transaction with market value													181,300,000	354,000,000	3,900,000,000	1,200,000,000
Carrying value of pledged securities owned that can be sold or re-pledged by the counterparty	391,900,000	570,000,000
Carrying value of securities owned by the Company loaned or pledged	141,800,000	159,400,000
Lehman Brothers International (Europe) held securities with a fair value																		7,500,000
Receivable from brokers and clearing organizations																			104,300,000
Guaranteed mortgages for a period												10 days
Outstanding under the warehouse facility line												2,800,000
Variable interest rate under warehouse facility																	1 month LIBOR plus 1.85%
Variable interest rate
Interest expense											$ 243,000						$ 219,600