Fair Value Measurement - Cash and Cash Equivalents (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Assets, Fair Value Disclosure
Total Assets, Fair Value	$ 74,364	$ 47,251	$ 22,896
Level 1 [Member]
Assets, Fair Value Disclosure
Total Assets, Fair Value	74,364	47,251	22,896
Level 2 [Member]
Assets, Fair Value Disclosure
Total Assets, Fair Value	0	0	0
Level 3 [Member]
Assets, Fair Value Disclosure
Total Assets, Fair Value	0	0	0
Money Market Funds [Member]
Assets, Fair Value Disclosure
Cash equivalents	68,765	37,284	22,896
Money Market Funds [Member] | Level 1 [Member]
Assets, Fair Value Disclosure
Cash equivalents	68,765	37,284	22,896
Money Market Funds [Member] | Level 2 [Member]
Assets, Fair Value Disclosure
Cash equivalents	0	0	0
Money Market Funds [Member] | Level 3 [Member]
Assets, Fair Value Disclosure
Cash equivalents	0	0	$ 0
Commercial paper [Member]
Assets, Fair Value Disclosure
Cash equivalents		9,967
Commercial paper [Member] | Level 1 [Member]
Assets, Fair Value Disclosure
Cash equivalents		9,967
Commercial paper [Member] | Level 2 [Member]
Assets, Fair Value Disclosure
Cash equivalents		0
Commercial paper [Member] | Level 3 [Member]
Assets, Fair Value Disclosure
Cash equivalents		$ 0
Restricted Cash [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	5,599
Restricted Cash [Member] | Level 1 [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	5,599
Restricted Cash [Member] | Level 2 [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	0
Restricted Cash [Member] | Level 3 [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	$ 0