Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 711.0	$ 737.7
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	408.1	406.0
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	141.1	159.2
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	51.4	43.5
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	41.6	48.3
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.2	37.6
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	33.6	43.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	468.2	489.2
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	263.1	260.4
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	141.1	159.2
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.5	21.9
IPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.0	0.0
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.4	23.5
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	18.1	24.2
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	242.8	248.5
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	145.0	145.6
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.9	21.6
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	41.6	48.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.8	14.1
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 15.5	$ 18.9