Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Regulatory assets
Current assets	$ 50.7	$ 37.2
Regulatory assets	3,805.1	2,803.2
Total regulatory assets	3,855.8	2,840.4
Allowance for return on equity capitalized for regulatory purposes	18.2	17.7
Regulatory assets not earning a return	125.4
Regulatory assets earning a return based on short-term interest rates	104.1
Regulatory assets earning a return based on long-term interest rates	316.7
Estimated future cash expenditures for environmental remediation	616.4	617.4
Pension and OPEB costs
Regulatory assets
Total regulatory assets	1,193.5	1,142.0
Plant retirements
Regulatory assets
Total regulatory assets	832.3	15.1
Environmental remediation costs
Regulatory assets
Total regulatory assets	687.1	676.6
Cash expenditures for environmental remediation costs	70.7
Estimated future cash expenditures for environmental remediation	616.4
Income tax related items
Regulatory assets
Total regulatory assets	369.1	15.7
System support resource (SSR)
Regulatory assets
Total regulatory assets	316.7	298.9
Asset retirement obligations (AROs)
Regulatory assets
Total regulatory assets	185.4	192.2
Electric transmission costs
Regulatory assets
Total regulatory assets	58.1	221.0
We Power generation
Regulatory assets
Total regulatory assets	43.0	71.3
Uncollectible expense
Regulatory assets
Total regulatory assets	38.7	35.1
Energy efficiency programs
Regulatory assets
Total regulatory assets	14.0	24.6
Other, net
Regulatory assets
Total regulatory assets	$ 117.9	$ 147.9