Commitments and Contingencies (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Oct. 31, 2001	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017
Commitment Contingencies And Guarantees [Line Items]
Other long-term liabilities		$ 3,106	$ 3,106			$ 3,535
Annual payments	$ 650
Severance costs		1,000	3,800
Payments for restructuring		1,000	1,500
Employee Severance
Commitment Contingencies And Guarantees [Line Items]
Maximum severance payments		5,400	5,400
Scenario, Forecast
Commitment Contingencies And Guarantees [Line Items]
Severance costs				$ 4,000
Letter of credit
Commitment Contingencies And Guarantees [Line Items]
Letter of credit under the credit facility					$ 6,300
Accrued expenses and other current liabilities
Commitment Contingencies And Guarantees [Line Items]
Liability to former chairman and chief executive officer, current		800	800
Other long-term liabilities
Commitment Contingencies And Guarantees [Line Items]
Liability to former chairman and chief executive officer, non-current		1,400	1,400
Accrued Compensation and Benefits
Commitment Contingencies And Guarantees [Line Items]
Severance costs			2,300
Clawback Liability
Commitment Contingencies And Guarantees [Line Items]
Other long-term liabilities		$ 300	$ 300
Company's ownership in the funds		19.00%	19.00%
Clawback liability paid							$ 1,000
Private equity funds
Commitment Contingencies And Guarantees [Line Items]
Company outstanding guarantees		$ 3,800	$ 3,800