Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 744.1	$ 821.2
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	406.2	421.7
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	179.5	243.0
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	53.0	64.3
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	33.8	15.4
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	31.4	19.4
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	40.2	57.4
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	502.5	583.5
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	261.3	279.1
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	179.5	243.0
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	22.9	15.1
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.8	19.4
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	23.0	26.9
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	241.6	237.7
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	144.9	142.6
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	53.0	64.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	10.9	0.3
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.6	0.0
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 17.2	$ 30.5