Commitments and Contingencies - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) Agreement	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Agreement Subsidiary	Sep. 30, 2016 USD ($)	Dec. 31, 2016 ft²
Loss Contingencies [Line Items]
Non-cancelable leases expiration date			Dec. 31, 2033
Rental expense			$ 4,100	$ 3,100
Number of lease agreements assigned to third parties | Agreement	1		1
Lease termination dates			2020-11
Future lease obligation under sublease arrangements	$ 900		$ 900
Number of subsidiaries | Subsidiary			2
Settlement days of bond transaction			within one to two trading days
Revenues from riskless principal transactions			$ 34,400	26,600
Collateral deposits	1,200	$ 1,100	1,200	1,100
Clearing expenses	1,476	$ 1,035	4,320	$ 4,754
Lease Agreement [Member]
Loss Contingencies [Line Items]
Newly built office space | ft²					83,000
Term of lease contract					15 years
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 900		$ 900