Operating Revenues Electric Utility Operating Revenues (Details) - Utility - Revenues from contracts with customers - Transferred over time - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 3,487.3	$ 3,617.8
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	3,088.3	3,212.7
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,822.1	2,918.1
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,206.7	1,220.8
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,010.9	1,020.0
Electric | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	583.9	656.6
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	20.6	20.7
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	93.8	108.5
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	132.7	153.7
Electric | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	23.3	24.1
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 16.4	$ 8.3