Short Term Investments - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	$ 24,793		$ 10,997
Interest rate on U.S. Treasury Bills (less than for 2017)			1.00%
US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	24,800		$ 11,000
U.S. Treasury Security, Maturing on March 14th 2019
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Interest rate on U.S. Treasury Bills		2.20%
U.S. Treasury Security, Maturing on March 14th 2019 | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	9,000
U.S. Treasury Security, Maturing on June 6th 2019
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Interest rate on U.S. Treasury Bills		2.40%
U.S. Treasury Security, Maturing on June 6th 2019 | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	10,900
U.S. Treasury Security, Maturing on June 20th 2019
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Interest rate on U.S. Treasury Bills		2.30%
U.S. Treasury Security, Maturing on June 20th 2019 | US Treasury Bill Securities
Securities Purchased Under Agreements to Resell and Other Short Term Investment Securities [Line Items]
Short-term investments	$ 4,900