Fair Value of Financial Assets and Liabilities - (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable securities:
Shares that warrant can purchase	7,614
Warrant purchase price	$ 3.94
Recurring
Assets:
Money market funds, included in cash and cash equivalents	$ 30,868	$ 114,593
Marketable securities:
U.S. Treasury obligations	143,441	60,576
Total assets	174,309	175,169
Recurring | Level 1
Assets:
Money market funds, included in cash and cash equivalents	30,868	114,593
Marketable securities:
U.S. Treasury obligations	143,441	60,576
Total assets	$ 174,309	$ 175,169