Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,371.0	$ 1,423.6
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	779.0	835.6
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	385.8	387.7
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	66.3	88.6
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	30.1	24.2
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	28.8	19.9
WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.4	21.9
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	60.6	45.7
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	717.9	714.7
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	343.6	350.9
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	256.4	257.0
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	30.7	51.3
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.0	8.8
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	26.3	17.4
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	46.9	29.3
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	653.1	708.9
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	435.4	484.7
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	129.4	130.7
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.6	37.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	16.1	15.4
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	2.5	2.5
WPL [Member] | WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.4	21.9
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 13.7	$ 16.4