Commitments and Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Agreement Subsidiary	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Loss Contingencies [Line Items]
Rental expense	$ 4.1	$ 3.8	$ 3.8
Number of lease agreements assigned to third parties | Agreement	1
Lease termination dates	2020-11
Future lease obligation under sublease arrangement	$ 1.4
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal trading	$ 16.9	7.4	6.0
Collateral deposits	1.5
General and Administrative Expense [Member]
Loss Contingencies [Line Items]
Clearing expenses	3.3	$ 1.3	$ 0.9
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 1.1