Commitments and Contingencies - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) Agreement	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Agreement Subsidiary	Jun. 30, 2017 USD ($)	Dec. 31, 2016 ft²
Loss Contingencies [Line Items]
Non-cancelable leases expiration date			Dec. 31, 2033
Rental expense			$ 6,300	$ 2,500
Number of lease agreements assigned to third parties | Agreement	1		1
Lease termination dates			2020-11
Future lease obligation under sublease arrangements	$ 600		$ 600
Number of subsidiaries | Subsidiary			2
Settlement days of bond transaction			Within one to two trading days
Revenues from riskless principal transactions			$ 28,700	23,200
Collateral deposits	1,200	$ 1,100	1,200	1,100
Clearing expenses	2,012	$ 1,517	3,737	$ 2,844
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	$ 1,700		$ 1,700