Operating Revenues - Other Operating Revenues (Details) - Other operating revenues - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Operating Revenues
Other operating revenues	$ 72.4	$ 73.8	$ (0.8)
Alternative revenues
Disaggregation of Operating Revenues
Other operating revenues	38.8	(9.6)	(45.6)
Late payment charges
Disaggregation of Operating Revenues
Other operating revenues	29.4	43.7	40.3
Other
Disaggregation of Operating Revenues
Other operating revenues	4.2	$ 39.7	$ 4.5
Late payment charges transferred to alternative revenues
Disaggregation of Operating Revenues
Other operating revenues	$ 8.5