Commitments and Contingencies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Agreement Subsidiary	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Non-cancelable leases expiration date	Dec. 31, 2033
Rental expense	$ 5,300	$ 4,000	$ 4,100
Number of lease agreements assigned to third parties | Agreement	1
Lease termination dates	2020-11
Future lease obligation under sublease arrangements	$ 800
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to two trading days
Revenues from riskless principal transactions	$ 45,700	37,700	16,900
Collateral deposits	1,200	1,100
Clearing expenses	5,797	$ 6,060	$ 3,313
Lease Agreement [Member]
Loss Contingencies [Line Items]
Newly built office space | ft²		83,000
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 900