Commitments and Contingencies - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) ft² Agreement Subsidiary	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Non-cancelable leases expiration date	Dec. 31, 2033
Rental expense	$ 3,100	$ 3,000
Aggregate minimum rental commitment under non-cancelable leases	$ 147,643
Number of lease agreements assigned to third parties | Agreement	1
Lease termination dates	2020-11
Future lease obligation under sublease arrangements	$ 1,100
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal transactions	$ 26,600	11,600
Collateral deposits	1,100		$ 1,500
General and Administrative Expense [Member]
Loss Contingencies [Line Items]
Clearing expenses	$ 4,800	$ 2,300
Non-Cancelable Leases Commence on or after December 1, 2016 [Member]
Loss Contingencies [Line Items]
Non-cancelable leases expiration date	Dec. 31, 2033
Non-cancelable leases commencement date	Dec. 01, 2016
Aggregate minimum rental commitment under non-cancelable leases	$ 127,400
Area of office space under non-cancelable leases | ft²	108,000
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 900