Commitments and Contingencies - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014 Subsidiary Agreement	Sep. 30, 2013
Loss Contingencies [Line Items]
Rental expense	$ 2,900,000	$ 3,000,000
Number of lease agreements assigned to third parties	2
Lease termination dates	November 2015 and November 2020
Future lease obligations under sublease arrangements	2,300,000
Possible range of loss	0
Provision for accrual loss	0
Number of subsidiaries	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal trading	4,700,000	4,500,000
Collateral deposit	900,000
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 1,300,000