Revenue - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Revenue from Contracts with Customers [Line Items]
Contract with Customer, Liability, Revenue Recognized	$ 2,402	$ 5,244
Period in Which Performance Obligations Under Client Arrangements Settled		1 year
Underwriting Fees [Member]
Revenue from Contracts with Customers [Line Items]
Reimbursement Revenue	1,677	$ 3,797
Professional Fees [Member]
Revenue from Contracts with Customers [Line Items]
Cost of Reimbursable Expense	838	1,923
Travel and Related Expenses [Member]
Revenue from Contracts with Customers [Line Items]
Cost of Reimbursable Expense	168	375
Communications and Information Services [Member]
Revenue from Contracts with Customers [Line Items]
Cost of Reimbursable Expense	168	375
Other Operating Expenses [Member]
Revenue from Contracts with Customers [Line Items]
Cost of Reimbursable Expense	$ 503	$ 1,124