Commitments and Contingencies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Agreement Subsidiary	Sep. 30, 2014 USD ($)
Loss Contingencies [Line Items]
Rental expense	$ 3.0	$ 2.9
Number of lease agreements assigned to third parties | Agreement	2
Lease termination dates	November 2015 and November 2020
Future lease obligations under sublease arrangements	$ 1.6
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal transactions	$ 11.6	4.7
Collateral deposit	1.0
General and Administrative Expense [Member]
Loss Contingencies [Line Items]
Clearing expenses	2.3	$ 0.9
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 1.3