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Summary of Significant Accounting Policies - Short-term investments (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Operating cycle        
Minimum liquidation period of assets in which significant working capital has been invested   1 year    
Goodwill impairment $ 2,700 $ 0 $ 0 $ 2,716
Income tax        
Estimated net tax benefit of Tax Act     $ 9,400  
Additional income tax related to complete accounting of Tax Act   1,100    
Additional income tax related to foreign tax credits   600    
Additional income tax related to depreciation   $ 500