Derivative Financial Instruments - Changes in Outstanding Warrants (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Fair Value of Derivative Liability [Roll Forward]
Balance at the beginning of the period		$ 17,582		$ 17,582
Change in fair value of warrants	$ (2,604)		$ (39)	(8,248)	$ (161)
Balance at the end of the period	$ 9,334			$ 9,334
Derivative instrument liability
Class of Warrant or Right [Roll Forward]
Balance of warrants outstanding, beginning of period (in shares)		21,915,426		21,915,426
Expiration of warrants (in shares)		(210,000)
Balance of warrants outstanding, end of period (in shares)	21,705,426			21,705,426
Derivative Fair Value of Derivative Liability [Roll Forward]
Balance at the beginning of the period		$ 17,582		$ 17,582
Change in fair value of warrants	$ (2,604)	$ (5,644)
Balance at the end of the period	$ 9,334			$ 9,334