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Revenue Recognition - Textuals (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Jun. 30, 2020
Dec. 31, 2018
Disaggregation of Revenue [Line Items]            
Interest Income $ 2.7 $ 2.1 $ 8.0 $ 6.3    
Accounts Receivable [Member]            
Disaggregation of Revenue [Line Items]            
Accounts Receivable, Allowance for Credit Loss 0.0   0.0     $ 0.1
Accounts Receivable, Credit Loss Expense (Reversal) 0.0 $ (0.4) (0.1) $ (0.2)    
Miscellaneous Receivables [Member]            
Disaggregation of Revenue [Line Items]            
Accounts Receivable, Allowance for Credit Loss 4.3   4.3     4.3
Long-term Customer Receivables            
Disaggregation of Revenue [Line Items]            
Long-term Investments and Receivables, Net $ 12.1   $ 12.1     $ 11.1
Scenario, Forecast | One year or longer            
Disaggregation of Revenue [Line Items]            
Deferred Revenue         $ 4,800.0  
Deferred Revenue, Percentage of Total, Expected to be Recognized in Next Twelve Months         46.00%  
Deferred Revenue, Percentage of Total, Expected to be Recognized Beyond Twelve Months         54.00%