Regulatory Matters (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 876.8	$ 821.5
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	422.7	418.9
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	266.3	265.4
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	67.5	52.7
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	41.2	7.2
IPL's electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.1	14.6
IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.7	21.6
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	51.3	41.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	623.2	614.9
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	280.9	277.7
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	266.3	265.4
IPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	8.6	9.3
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	8.4	3.6
IPL [Member] | IPL's electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.1	14.6
IPL [Member] | IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.7	21.6
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	31.2	22.7
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	253.6	206.6
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	141.8	141.2
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	58.9	43.4
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	32.8	3.6
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 20.1	$ 18.4