Regulatory Matters (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 821.2	$ 821.5
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	421.7	418.9
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	243.0	265.4
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	64.3	52.7
IPL's electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.4	14.6
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.4	7.5
IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11.3	21.6
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	46.1	40.8
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	583.5	614.9
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	279.1	277.7
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	243.0	265.4
IPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	9.3
IPL [Member] | IPL's electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.4	14.6
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.1	5.5
IPL [Member] | IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11.3	21.6
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.6	20.8
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	237.7	206.6
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	142.6	141.2
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	64.3	43.4
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.3	2.0
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 30.5	$ 20.0