Fair Value Measurements - Fair Value of Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities related to Warrants	$ 17,582	$ 216
Derivative instrument liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities related to Warrants	17,582	216	$ 322
Fair value on a recurring basis | Warrants | Derivative instrument liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities related to Warrants	17,582	216
Fair value on a recurring basis | Warrants | Derivative instrument liability | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities related to Warrants	0	0
Fair value on a recurring basis | Warrants | Derivative instrument liability | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities related to Warrants	0	0
Fair value on a recurring basis | Warrants | Derivative instrument liability | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities related to Warrants	$ 17,582	$ 216