Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets
Total assets	$ 114,700	$ 112,823
Fair Value, Inputs, Level 1
Assets
Total assets	114	1,156
Fair Value, Inputs, Level 2
Assets
Total assets	114,586	111,667
Fair Value, Measurements, Recurring
Liabilities
Total liabilities	0	0
Money market funds
Assets
Total assets	114	1,156
Money market funds | Fair Value, Inputs, Level 1
Assets
Total assets	114	1,156
U.S. Government agency bonds
Assets
Total assets	500	1,982
U.S. Government agency bonds | Fair Value, Inputs, Level 2
Assets
Total assets	500	1,982
U.S. Government bonds
Assets
Total assets		1,297
U.S. Government bonds | Fair Value, Inputs, Level 2
Assets
Total assets		1,297
Bank certificates of deposit
Assets
Cash equivalents	1,201
Total assets	11,713	15,201
Bank certificates of deposit | Fair Value, Inputs, Level 2
Assets
Total assets	11,713	15,201
Commercial paper.
Assets
Total assets	9,454	9,593
Commercial paper. | Fair Value, Inputs, Level 2
Assets
Total assets	9,454	9,593
Corporate notes
Assets
Cash equivalents		1,000
Total assets	67,026	61,752
Corporate notes | Fair Value, Inputs, Level 2
Assets
Total assets	67,026	61,752
Asset-backed securities
Assets
Total assets	25,893	21,842
Asset-backed securities | Fair Value, Inputs, Level 2
Assets
Total assets	$ 25,893	$ 21,842