Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,442.9	$ 1,493.2
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	853.8	890.6
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	399.2	401.2
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	101.5	104.0
WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.2	25.4
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	22.7	16.8
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	40.5	55.2
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	719.9	754.9
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	365.6	390.1
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	269.5	273.3
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	52.7	47.7
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11.9	11.9
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.2	31.9
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	723.0	738.3
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	488.2	500.5
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	129.7	127.9
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	48.8	56.3
WPL [Member] | WPL's earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.2	25.4
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	10.8	4.9
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 20.3	$ 23.3