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Fair Value Measurement of Assets and Liabilities - Other Financial Instruments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Convertible Debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Principal $ 105,125 $ 120,000
Level 3 | Fair Value, Measurements, Nonrecurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Convertible debt, fair value $ 84,800 $ 90,900