Commitments and Contingencies - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) Agreement Subsidiary	Mar. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Rental expense	$ 1.0	$ 0.9
Number of lease agreements assigned to third parties | Agreement	1
Lease termination dates	2020-11
Future lease obligation under sublease arrangements	$ 1.4
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal transactions	$ 7.5	3.5
Collateral deposits	1.5
General and Administrative Expense [Member]
Loss Contingencies [Line Items]
Clearing expenses	1.8	$ 0.7
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 1.1