Operating Revenues - Disaggregation of 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	$ 860.1	$ 884.1	$ 791.7	$ 960.8	$ 903.3	$ 924.0	$ 856.2	$ 941.5	$ 3,496.7	$ 3,625.0	$ 3,711.7
Utility
Disaggregation of Operating Revenues
Operating revenues									3,496.7	3,625.0	$ 3,711.7
Utility | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									9.4	7.2
Utility | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									3,487.3	3,617.8
Utility | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									3,088.3	3,212.7
Utility | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									$ 399.0	$ 405.1