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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                      
Net operating revenues $ 1,096.0 $ 1,067.6 $ 1,067.7 $ 1,046.0 $ 1,008.8 $ 981.6 $ 966.4 $ 957.1 $ 4,277.3 $ 3,913.9 $ 3,642.6
Other operating expenses                 $ 585.1 531.6 490.6
Adjustments for New Accounting Pronouncement [Member]                      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                      
Net operating revenues                   3,913.9 3,642.6
Less: Provision for doubtful accounts                   0.0 0.0
Other operating expenses                   531.6 488.7
Adjustments for ASC 606 [Member]                      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                      
Net operating revenues                   (57.5) (64.6)
Less: Provision for doubtful accounts                   (52.4) (61.2)
Other operating expenses                   (5.1) (3.4)
As Reported [Member]                      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                      
Net operating revenues                   3,971.4 3,707.2
Less: Provision for doubtful accounts                   52.4 61.2
Other operating expenses                   $ 536.7 $ 492.1