Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,423.6	$ 1,493.2
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	835.6	890.6
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	387.7	401.2
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	88.6	104.0
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	24.2	16.8
WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.9	25.4
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.9	18.5
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	45.7	36.7
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	714.7	754.9
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	350.9	390.1
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	257.0	273.3
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	51.3	47.7
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	8.8	11.9
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	17.4	10.2
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	29.3	21.7
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	708.9	738.3
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	484.7	500.5
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	130.7	127.9
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	37.3	56.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.4	4.9
WPL [Member] | WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.9	25.4
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	2.5	8.3
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 16.4	$ 15.0