Fair Value Measurements - Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Assets, fair value	$ 56,878	$ 75,476
Level 1
Assets:
Assets, fair value	4,831	5,838
Level 2
Assets:
Assets, fair value	52,047	69,638
Level 3
Assets:
Assets, fair value	0	0
Money market funds
Assets:
Assets, fair value	4,831	5,838
Money market funds | Level 1
Assets:
Assets, fair value	4,831	5,838
Money market funds | Level 2
Assets:
Assets, fair value	0	0
Money market funds | Level 3
Assets:
Assets, fair value	0	0
Repurchase agreements
Assets:
Assets, fair value	0	5,000
Repurchase agreements | Level 1
Assets:
Assets, fair value	0	0
Repurchase agreements | Level 2
Assets:
Assets, fair value	0	5,000
Repurchase agreements | Level 3
Assets:
Assets, fair value	0	0
Commercial paper
Assets:
Assets, fair value	17,149	30,710
Commercial paper | Level 1
Assets:
Assets, fair value	0	0
Commercial paper | Level 2
Assets:
Assets, fair value	17,149	30,710
Commercial paper | Level 3
Assets:
Assets, fair value	0	0
Corporate notes
Assets:
Assets, fair value	17,245	13,144
Corporate notes | Level 1
Assets:
Assets, fair value	0	0
Corporate notes | Level 2
Assets:
Assets, fair value	17,245	13,144
Corporate notes | Level 3
Assets:
Assets, fair value	0	0
U.S. government securities
Assets:
Assets, fair value	6,966	10,166
U.S. government securities | Level 1
Assets:
Assets, fair value	0	0
U.S. government securities | Level 2
Assets:
Assets, fair value	6,966	10,166
U.S. government securities | Level 3
Assets:
Assets, fair value	0	0
U.S. asset-backed securities
Assets:
Assets, fair value	10,687	10,618
U.S. asset-backed securities | Level 1
Assets:
Assets, fair value	0	0
U.S. asset-backed securities | Level 2
Assets:
Assets, fair value	10,687	10,618
U.S. asset-backed securities | Level 3
Assets:
Assets, fair value	$ 0	$ 0