Commitments and Contingencies (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	Jan. 31, 2018	Oct. 31, 2001	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Commitment Contingencies And Guarantees [Line Items]
Total rental expense under operating leases				$ 600	$ 500
2018				605
2019				603
2020				598
2021				602
2023 and thereafter				2,046
Future minimum payments				4,454
Company outstanding guarantees				3,800
Accrued expenses				2,182	$ 1,851
Annual payments		$ 650
Severance costs	$ 3,900			2,800
Employee Severance [Member]
Commitment Contingencies And Guarantees [Line Items]
Restructuring and related cost, expected cost				4,800
Accrued expenses and other current liabilities [Member]
Commitment Contingencies And Guarantees [Line Items]
Liability to former chairman and chief executive officer, current				800
Other long-term liabilities [Member]
Commitment Contingencies And Guarantees [Line Items]
Liability to former chairman and chief executive officer, non-current				$ 1,300
Clawback Liability [Member]
Commitment Contingencies And Guarantees [Line Items]
Company's ownership in the funds				19.00%
Accrued expenses				$ 1,000
Estimate of possible loss				$ 300
Scenario, Forecast [Member]
Commitment Contingencies And Guarantees [Line Items]
Severance costs			$ 1,200