Short-term Investments - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	$ 24,831	$ 24,793
US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	24,800	24,800
U.S. Treasury Security, Maturing on March 14th 2019 [Member] | 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%
U.S. Treasury Security, Maturing on June 6th, 2019 [Member] | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	$ 10,900	$ 10,900
Interest rate on U.S. Treasury Bills	2.40%	2.40%
U.S. Treasury Security, Maturing on June 20th, 2019 [Member] | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	$ 4,900	$ 4,900
Interest rate on U.S. Treasury Bills	2.30%	2.30%
U.S. Treasury Security, Maturing on September 12 2019 [Member] | 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	2.30%