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Investments - Additional Information (Details) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Long-term Investments $ 0 $ 0
Short-term investments 148,371,000 $ 0
Short-term investments classified as cash equivalents 11,400,000  
Short Term Investments    
Schedule Of Available For Sale Securities [Line Items]    
Fair value of short-term investments in an unrealized loss position $ 114,100,000