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Investments - Other Investment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Other Investment      
Impairment charges for investments $ 0.0 $ 0.0 $ 0.0
Kateeva      
Other Investment      
Carrying amount $ 21.0 $ 21.0  
Kateeva | Maximum      
Other Investment      
Percentage ownership of cost method investee 20.00%    
Separate non-marketable investment      
Other Investment      
Amount of investment made $ 3.5    
Separate non-marketable investment | Maximum      
Other Investment      
Percentage ownership of cost method investee 20.00%