Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Subsidiary Agreement	Dec. 31, 2013	Dec. 31, 2012
Loss Contingencies [Line Items]
Rental expense	$ 3.8	$ 3.8	$ 2.1
Number of lease agreements assigned to third parties	2
Lease termination dates	November 2015 and November 2020
Future lease obligations under sublease arrangements	2.1
Number of subsidiaries	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal trading	7.4	6.0	3.5
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