Commitments and Contingencies - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2016 USD ($) Agreement Subsidiary	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Rental expense	$ 2.1	$ 1.9
Number of lease agreements assigned to third parties | Agreement	1
Lease termination dates	2020-11
Future lease obligation under sublease arrangements	$ 1.2
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal transactions	$ 16.4	7.4
Collateral deposits	1.4		$ 1.5
General and Administrative Expense [Member]
Loss Contingencies [Line Items]
Clearing expenses	3.7	$ 1.4
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 0.9