Fair Value Measurements - Assets measured at fair value on a recurring basis (Details) - Fair value on recurring basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurement, Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Available-for-sale Securities [Line Items]
Total	$ 50,148	$ 49,986
Fair Value Measurement, Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds (included in cash and cash equivalents)
Schedule of Available-for-sale Securities [Line Items]
Money market funds (included in cash and cash equivalents)	50,148	49,986
Fair Value Measurement, Significant Other Observable Inputs (Level 2)
Schedule of Available-for-sale Securities [Line Items]
Total	0	0
Fair Value Measurement, Significant Other Observable Inputs (Level 2) | Money market funds (included in cash and cash equivalents)
Schedule of Available-for-sale Securities [Line Items]
Money market funds (included in cash and cash equivalents)	0	0
Fair Value Measurement, Significant Unobservable Inputs (Level 3)
Schedule of Available-for-sale Securities [Line Items]
Total	0	0
Fair Value Measurement, Significant Unobservable Inputs (Level 3) | Money market funds (included in cash and cash equivalents)
Schedule of Available-for-sale Securities [Line Items]
Money market funds (included in cash and cash equivalents)	$ 0	$ 0