Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,393.9	$ 1,423.6
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	799.4	835.6
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	387.9	387.7
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	97.4	88.6
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.8	24.2
WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.4	21.9
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	16.4	19.9
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	46.6	45.7
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	720.8	714.7
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	345.0	350.9
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	258.1	257.0
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	62.0	51.3
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	10.7	8.8
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.8	17.4
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	30.2	29.3
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	673.1	708.9
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	454.4	484.7
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	129.8	130.7
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.4	37.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.1	15.4
WPL [Member] | WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.4	21.9
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	1.6	2.5
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 16.4	$ 16.4