Fair Value Measurements - Reconciliation of Warrant Liability Included in Expenses and Other Current Liabilities (Details) - Warrant - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of warrant liability and embedded derivative
Balance at beginning of period	$ 1,226	$ 3,072
Issuance of common stock warrants	0	858
Exercise of common stock warrants	(34)	(1,150)
Remeasurement of common stock warrants	(523)	(1,554)
Balance at end of period	$ 669	$ 1,226