Significant Accounting Policies - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents			$ 20,751		$ 22,058
Marketable securities			4,500
Cash, cash equivalents, and short-term investments			25,200
Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Severance costs	$ 1,300
Revolving Credit Facility [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Maximum borrowing capacity				$ 75,000
Outstanding debentures				50,000
Liquidity threshold				$ 20,000
Tangible net worth plus unrestricted cash multiplier				1.75
Minimum aggregate appraised value plus liquidity threshold				$ 350,000
Convertible Senior Debentures due 2018 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Long term debt, principal amount outstanding			41,000
Credit Facility due May 2020 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Long term debt, principal amount outstanding			50,000
Current borrowing capacity			$ 25,000
Minimum [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restructuring, projected annual cost savings		$ 5,000
Maximum [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restructuring, projected annual cost savings		$ 6,000