Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Jan. 01, 2019	May 31, 2018	Dec. 31, 2017	May 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents			$ 7,703			$ 20,751
Marketable securities			38,000
Cash, cash equivalents, and short-term investments			45,700
Long-term Debt			68,600
Severance costs	$ 3,900		$ 2,800
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Liability				$ 2,900
Operating Lease, Right-of-Use Asset				2,200
Deferred Rent Credit				$ 700
Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Severance costs		$ 1,200
Revolving Credit Facility [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Maximum borrowing capacity					$ 100,000		$ 75,000
Outstanding debentures					68,600
Liquidity threshold					20,000
Minimum aggregate appraised value plus liquidity threshold					350,000
Debt Instrument, Debt Covenant, Expense Threshold					11,500
Debt Instrument, Debt Covenants, Cash and Cash Equivalent Threshold					$ 50,000
Debt Instrument, Debt Covenants, Liquidity Threshold, Percent					100.00%