Operating Revenues - Other Operating Revenues (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
Disaggregation of Operating Revenues
Operating revenues	$ 2,076.8	$ 1,643.7	$ 1,672.5	$ 2,286.5	$ 2,055.0	$ 1,657.5	$ 1,631.5	$ 2,304.5	$ 7,679.5	$ 7,648.5	$ 7,472.3
Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									(0.8)
Other operating revenues \| Alternative revenues
Disaggregation of Operating Revenues
Operating revenues									(45.6)
Other operating revenues \| Late payment charges
Disaggregation of Operating Revenues
Operating revenues									40.3
Other operating revenues \| Leases
Disaggregation of Operating Revenues
Operating revenues									$ 4.5