Commitments and Contingencies - Additional Information (Detail) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) Agreement Subsidiary	Sep. 30, 2017 USD ($)	Dec. 31, 2016 ft²
Loss Contingencies [Line Items]
Non-cancelable leases expiration date	Dec. 31, 2033
Rental expense	$ 9.6	$ 4.1
Number of lease agreements assigned to third parties | Agreement	1
Lease termination dates	2020-11
Future lease obligation under sublease arrangements	$ 0.6
Number of subsidiaries | Subsidiary	2
Settlement days of bond transaction	Within one to two trading days
Collateral deposits	$ 1.2	$ 1.2
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	$ 1.7