Commitments and Contingencies - Additional Information (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) Agreement Subsidiary	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) ft²
Loss Contingencies [Line Items]
Non-cancelable leases expiration date	Dec. 31, 2033
Rental expense	$ 1,200	$ 1,000
Aggregate minimum rental commitment remaining under non-cancelable leases	$ 145,378
Number of lease agreements assigned to third parties | Agreement	1
Lease termination dates	2020-11
Future lease obligation under sublease arrangements	$ 900
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal transactions	$ 11,900	7,500
Collateral deposits	1,100	1,500
Clearing expenses	1,327	$ 1,766
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			$ 127,200
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 900