Fair Value Measurements - Summary of Fair Value Measurements of Company's Financial Assets and Liabilities (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Assets at fair value	$ 119,486	$ 164,356
Liabilities:
Derivative liability	594	585
Level 1 [Member]
Assets:
Assets at fair value	89,758	77,234
Level 2 [Member]
Assets:
Assets at fair value	29,728	87,122
Liabilities:
Derivative liability		52
Level 3 [Member]
Liabilities:
Derivative liability	594	533
Derivative Liability Exit Fee [Member]
Liabilities:
Derivative liability	594	533
Derivative Liability Exit Fee [Member] | Level 3 [Member]
Liabilities:
Derivative liability	594	533
Foreign Currency Derivative Contracts [Member]
Liabilities:
Derivative liability		52
Foreign Currency Derivative Contracts [Member] | Level 2 [Member]
Liabilities:
Derivative liability		52
Money Market Funds [Member]
Assets:
Assets at fair value	85,171	73,238
Money Market Funds [Member] | Level 1 [Member]
Assets:
Assets at fair value	85,171	73,238
U.S. Treasury Notes [Member]
Assets:
Assets at fair value	2,293
U.S. Treasury Notes [Member] | Level 1 [Member]
Assets:
Assets at fair value	2,293
U.S. Treasury Securities [Member]
Assets:
Assets at fair value	2,294	3,996
U.S. Treasury Securities [Member] | Level 1 [Member]
Assets:
Assets at fair value	2,294	3,996
Corporate Bonds [Member]
Assets:
Assets at fair value	7,748	34,590
Corporate Bonds [Member] | Level 2 [Member]
Assets:
Assets at fair value	7,748	34,590
Commercial Paper [Member]
Assets:
Assets at fair value	17,679	43,154
Commercial Paper [Member] | Level 2 [Member]
Assets:
Assets at fair value	17,679	43,154
Asset-Backed Securities [Member]
Assets:
Assets at fair value	4,301	9,378
Asset-Backed Securities [Member] | Level 2 [Member]
Assets:
Assets at fair value	$ 4,301	$ 9,378