Credit Losses - Gross Receivables and Related Allowances (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,422.9
Allowance for credit losses	220.1	$ 140.0
Accounts receivable and unbilled revenues, net	1,202.8
Total accounts receivable, net - past due greater than 90 days	$ 122.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.50%
Amount of net accounts receivable with regulatory protections	$ 679.4
Percent of net accounts receivable with regulatory protections	56.50%
Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,373.5
Allowance for credit losses	220.1	139.9
Accounts receivable and unbilled revenues, net	1,153.4
Total accounts receivable, net - past due greater than 90 days	$ 122.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.50%
Wisconsin | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 899.8
Allowance for credit losses	102.1	59.9
Accounts receivable and unbilled revenues, net	797.7
Total accounts receivable, net - past due greater than 90 days	$ 84.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	97.60%
Illinois | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 393.9
Allowance for credit losses	111.6	75.9
Accounts receivable and unbilled revenues, net	282.3
Total accounts receivable, net - past due greater than 90 days	$ 34.5
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	100.00%
Other States | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 79.8
Allowance for credit losses	6.4	4.1
Accounts receivable and unbilled revenues, net	73.4
Total accounts receivable, net - past due greater than 90 days	$ 3.5
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%
Non-Utility Energy Infrastructure
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 45.0
Allowance for credit losses	0.0
Accounts receivable and unbilled revenues, net	45.0
Total accounts receivable, net - past due greater than 90 days	$ 0.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%
Corporate and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 4.4
Allowance for credit losses	0.0	$ 0.1
Accounts receivable and unbilled revenues, net	4.4
Total accounts receivable, net - past due greater than 90 days	$ 0.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%