Fair Value Measurement Cash and Cash Equivalents (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Assets, Fair Value Disclosure
Total Assets, Fair Value	$ 4,891	$ 47,251	$ 28,091
Level 1
Assets, Fair Value Disclosure
Total Assets, Fair Value	4,891	47,251	28,091
Level 2
Assets, Fair Value Disclosure
Total Assets, Fair Value	0	0	0
Level 3
Assets, Fair Value Disclosure
Total Assets, Fair Value	0	0	0
Money Market Funds
Assets, Fair Value Disclosure
Cash equivalents	4,891	37,284	28,091
Money Market Funds | Level 1
Assets, Fair Value Disclosure
Cash equivalents	4,891	37,284	28,091
Money Market Funds | Level 2
Assets, Fair Value Disclosure
Cash equivalents	0	0	0
Money Market Funds | Level 3
Assets, Fair Value Disclosure
Cash equivalents	$ 0	0	$ 0
Commercial paper
Assets, Fair Value Disclosure
Cash equivalents		9,967
Commercial paper | Level 1
Assets, Fair Value Disclosure
Cash equivalents		9,967
Commercial paper | Level 2
Assets, Fair Value Disclosure
Cash equivalents		0
Commercial paper | Level 3
Assets, Fair Value Disclosure
Cash equivalents		$ 0