Operating Revenues - Other Operating Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Late payment charges transferred to alternative revenues
Disaggregation of Operating Revenues
Other operating revenues		$ 8.5
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	$ 80.3	72.4	$ 73.8
Other operating revenues | Late payment charges
Disaggregation of Operating Revenues
Other operating revenues	54.9	29.4	43.7
Other operating revenues | Alternative revenues
Disaggregation of Operating Revenues
Other operating revenues	21.2	38.8	(9.6)
Other operating revenues | Other
Disaggregation of Operating Revenues
Other operating revenues	$ 4.2	$ 4.2	$ 39.7