Fair Value Measurement Cash and Cash Equivalents (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Assets, Fair Value Disclosure
Total Assets, Fair Value	$ 38,006	$ 10,057	$ 35,076
Level 1
Assets, Fair Value Disclosure
Total Assets, Fair Value	38,006	10,057	35,076
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	38,006	10,057	30,082
Money Market Funds | Level 1
Assets, Fair Value Disclosure
Cash equivalents	38,006	10,057	30,082
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 [Member]
Assets, Fair Value Disclosure
Cash equivalents			4,994
Commercial Paper [Member] | Level 1
Assets, Fair Value Disclosure
Cash equivalents			4,994
Commercial Paper [Member] | Level 2
Assets, Fair Value Disclosure
Cash equivalents			0
Commercial Paper [Member] | Level 3
Assets, Fair Value Disclosure
Cash equivalents			$ 0