Operating Revenues - Other Operating Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Operating Revenues
Operating revenues	$ 1,947.5	$ 1,608.0	$ 1,590.2	$ 2,377.4	$ 2,076.8	$ 1,643.7	$ 1,672.5	$ 2,286.5	$ 7,523.1	$ 7,679.5	$ 7,648.5
Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									73.8	(0.8)
Other operating revenues | Late payment charges
Disaggregation of Operating Revenues
Operating revenues									43.7	40.3
Other operating revenues | Alternative revenues
Disaggregation of Operating Revenues
Operating revenues									(9.6)	(45.6)
Other operating revenues | Other
Disaggregation of Operating Revenues
Operating revenues									$ 39.7	$ 4.5