General Significant Accounting Policies (Details)	Jun. 30, 2017 USD ($)	May 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 30, 2012 USD ($)
Debt Instrument [Line Items]
Marketable securities	$ 5,400,000
Cash, cash equivalents, and marketable securities	53,600,000
Cash and cash equivalents	48,181,000		$ 22,058,000
Long-term restricted cash equivalents	6,336,000		6,336,000
Total cash, cash equivalents and restricted cash equivalents	54,517,000		$ 28,394,000	$ 66,231,000	$ 32,838,000
Convertible Debt [Member]
Debt Instrument [Line Items]
Principal amount outstanding	43,600,000
Debt instrument, face amount	43,600,000					$ 55,000,000.0
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 75,000,000
Debt instrument, face amount	$ 50,000,000.0
Liquidity threshold		20,000,000
Unrestricted cash multiplier	1.75
Minimum aggregate appraised value plus liquidity threshold		$ 350,000,000