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Revenue Recognition - Textuals (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2020
Dec. 31, 2018
Disaggregation of Revenue [Line Items]            
Interest Income $ 2.6 $ 2.1 $ 5.3 $ 4.2    
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) (0.1) $ 0.0 (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 $ 11.3   $ 11.3     $ 11.1
Scenario, Forecast | One year or longer            
Disaggregation of Revenue [Line Items]            
Deferred Revenue         $ 5,400.0  
Deferred Revenue, Percentage of Total, Expected to be Recognized in Next Twelve Months         45.00%  
Deferred Revenue, Percentage of Total, Expected to be Recognized Beyond Twelve Months         55.00%