Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,512.9	$ 1,497.2
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	908.4	899.4
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	402.4	410.0
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	105.6	90.4
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.7	21.0
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.8	25.0
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	51.0	51.4
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	774.8	755.4
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	406.8	399.5
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	274.9	274.5
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	41.1	26.4
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.4	14.6
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.8	25.0
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11.8	15.4
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	738.1	741.8
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	501.6	499.9
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	127.5	135.5
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	64.5	64.0
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.3	6.4
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 39.2	$ 36.0