General Significant Accounting Policies (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Cash and cash equivalents	$ 31,294,000		$ 20,751,000
Marketable securities	3,000,000
Cash, cash equivalents, and marketable securities	34,300,000
Severance costs	1,100,000
Cash and cash equivalents	31,294,000		20,751,000
Long-term restricted cash equivalents	0		6,336,000
Total cash, cash equivalents and restricted cash equivalents	31,294,000		27,087,000		$ 36,879,000	$ 28,394,000
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 75,000,000
Long-term debt, gross	50,000,000
Liquidity threshold				$ 20,000,000
Unrestricted cash multiplier				1.75
Minimum aggregate appraised value plus liquidity threshold				$ 350,000,000
Scenario, Forecast
Debt Instrument [Line Items]
Severance costs		$ 3,800,000
Scenario, Forecast | Minimum
Debt Instrument [Line Items]
Projected annual cost savings		8,000,000
Scenario, Forecast | Maximum
Debt Instrument [Line Items]
Projected annual cost savings		$ 9,000,000
Convertible Senior Debentures due 2018
Debt Instrument [Line Items]
Principal amount outstanding	$ 41,000,000
Credit Facility due May 2020
Debt Instrument [Line Items]
Principal amount outstanding			50,000,000
Current borrowing capacity			$ 25,000,000