Fair Value Measurement Cash and Cash Equivalents (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Assets, Fair Value Disclosure
Total Assets, Fair Value	$ 62,280	$ 47,251	$ 38,006
Level 1 [Member]
Assets, Fair Value Disclosure
Total Assets, Fair Value	62,280	47,251	38,006
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	56,534	37,284	38,006
Money Market Funds [Member] | Level 1 [Member]
Assets, Fair Value Disclosure
Cash equivalents	56,534	37,284	38,006
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	5,746
Restricted Cash [Member] | Level 1 [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	5,746
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