Commitments and Contingencies - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2015 USD ($) Agreement Subsidiary	Jun. 30, 2014 USD ($)
Loss Contingencies [Line Items]
Rental expense	$ 1.9	$ 1.9
Number of lease agreements assigned to third parties | Agreement	2
Lease termination dates	November 2015 and November 2020
Future lease obligations under sublease arrangements	$ 1.9
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction
Revenues from riskless principal transactions	$ 7.4	$ 2.8
Collateral deposit	0.9
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 1.3