Fair Value Measurements - Assets and Liabilities at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	$ 12,489
Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	52,373	$ 71,431
Fair value on a recurring basis | Cash
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Cash and Money market funds	250
Other cash equivalents and Short-term investments		8,446
Fair value on a recurring basis | Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Cash and Money market funds	36,647	61,177
Fair value on a recurring basis | Other cash equivalents – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	2,987	1,808
Fair value on a recurring basis | Short-term investments – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	3,496
Fair value on a recurring basis | Short-term investments – U.S. treasury bills
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	8,993
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	45,890	69,623
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis | Cash
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Cash and Money market funds	250
Other cash equivalents and Short-term investments		8,446
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis | Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Cash and Money market funds	36,647	61,177
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis | Short-term investments – U.S. treasury bills
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	8,993
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	6,483	1,808
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis | Other cash equivalents – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	2,987	$ 1,808
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis | Short-term investments – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	$ 3,496