Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,505.6	$ 1,497.2
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	891.1	899.4
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	401.0	410.0
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	97.0	90.4
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.1	21.0
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.0	25.0
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	83.4	51.4
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	762.6	755.4
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	389.9	399.5
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	274.0	274.5
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	37.9	26.4
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.6	14.6
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.0	25.0
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	34.2	15.4
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	743.0	741.8
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	501.2	499.9
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	127.0	135.5
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	59.1	64.0
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.5	6.4
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 49.2	$ 36.0