Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Total assets	$ 115,571	$ 115,284
Liabilities
Cash-settled stock options	2,165	6,685
Total liabilities	2,165	6,685
Fair Value, Inputs, Level 1
Assets
Total assets	3,975	2,530
Fair Value, Inputs, Level 2
Assets
Total assets	111,596	112,754
Liabilities
Cash-settled stock options	2,165	6,685
Total liabilities	2,165	6,685
Money market funds
Assets
Total assets	3,975	2,530
Money market funds | Fair Value, Inputs, Level 1
Assets
Total assets	3,975	2,530
U.S. Government agency bonds
Assets
Total assets	1,503
U.S. Government agency bonds | Fair Value, Inputs, Level 2
Assets
Total assets	1,503
Bank certificates of deposit
Assets
Cash equivalents	1,500	1,201
Total assets	13,909	14,208
Bank certificates of deposit | Fair Value, Inputs, Level 2
Assets
Total assets	13,909	14,208
Commercial paper.
Assets
Total assets	8,961	7,484
Commercial paper. | Fair Value, Inputs, Level 2
Assets
Total assets	8,961	7,484
Corporate notes
Assets
Total assets	63,573	65,638
Corporate notes | Fair Value, Inputs, Level 2
Assets
Total assets	63,573	65,638
Asset-backed securities
Assets
Total assets	23,650	25,424
Asset-backed securities | Fair Value, Inputs, Level 2
Assets
Total assets	$ 23,650	$ 25,424