Short-term Investments - Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	$ 24,868	$ 24,793
US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	24,900	24,800
U.S. Treasury Security, Maturing on March 12, 2020 | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	$ 8,900
Interest rate on U.S. Treasury Bills	1.80%
Interest rate on U.S. Treasury Bills	1.80%
U.S. Treasury Security, Maturing on December 12, 2019 | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	$ 16,000
Interest rate on U.S. Treasury Bills	1.90%
Interest rate on U.S. Treasury Bills	1.90%
U.S. Treasury Security, Maturing on March 14, 2019 | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments		$ 9,000
Interest rate on U.S. Treasury Bills		2.20%
Interest rate on U.S. Treasury Bills		2.20%
U.S. Treasury Security, Maturing on June 6, 2019 | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments		$ 10,900
Interest rate on U.S. Treasury Bills		2.40%
Interest rate on U.S. Treasury Bills		2.40%
U.S. Treasury Security, Maturing on June 20, 2019 | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments		$ 4,900
Interest rate on U.S. Treasury Bills		2.30%
Interest rate on U.S. Treasury Bills		2.30%