Fair Value Measurements - Recurring (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Money market funds
Fair Value Measurements
Assets, fair value	$ 6,270	$ 10,601
U.S. government agencies
Fair Value Measurements
Assets, fair value	1,200	1,201
Commercial paper
Fair Value Measurements
Assets, fair value	800	6,589
Level 1 | Money market funds
Fair Value Measurements
Assets, fair value	6,270	10,601
Level 2 | U.S. government agencies
Fair Value Measurements
Assets, fair value	1,200	1,201
Level 2 | Commercial paper
Fair Value Measurements
Assets, fair value	$ 800	$ 6,589