Commitments and Contingencies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) ft² Agreement Subsidiary	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Loss Contingencies [Line Items]
Non-cancelable leases expiration date	Dec. 31, 2033
Rental expense	$ 4,000	$ 4,100	$ 3,800
Aggregate minimum rental commitment under non-cancelable leases	$ 146,325
Number of lease agreements assigned to third parties | Agreement	1
Lease termination dates	2020-11
Future lease obligation under sublease arrangements	$ 1,000
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal transactions	$ 37,700	16,900	7,400
Collateral deposits	1,100
General and Administrative Expense [Member]
Loss Contingencies [Line Items]
Clearing expenses	6,100	$ 3,300	$ 1,300
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 900
Non-Cancelable Leases Commence on or after December 1, 2016 [Member]
Loss Contingencies [Line Items]
Non-cancelable leases expiration date	Dec. 31, 2033
Area of office space under non-cancelable leases | ft²	108,000
Non-cancelable leases commencement date	Dec. 01, 2016
Aggregate minimum rental commitment under non-cancelable leases	$ 127,400