Commitments and Contingencies - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) Agreement	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Agreement Subsidiary	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) ft²
Loss Contingencies [Line Items]
Non-cancelable leases expiration date			Dec. 31, 2033
Rental expense			$ 2,500	$ 2,100
Aggregate minimum rental commitment remaining under non-cancelable leases	$ 144,646		$ 144,646
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 three trading days
Revenues from riskless principal transactions			$ 23,200	16,400
Collateral deposits	1,100	$ 1,400	1,100	1,400
Clearing expenses	1,517	$ 1,953	2,844	$ 3,719
Non-Cancelable Leases Commence on or after December 1, 2016 [Member]
Loss Contingencies [Line Items]
Non-cancelable leases expiration date					Dec. 31, 2033
Area of office space under non-cancelable leases | ft²					108,000
Non-cancelable leases commencement date					Dec. 01, 2016
Aggregate minimum rental commitment remaining under non-cancelable leases					$ 126,800
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 1,000		$ 1,000