Regulatory Matters (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 863.5	$ 821.5
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	422.3	418.9
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	243.3	265.4
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	67.1	52.7
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	48.2	7.2
IPL's electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.8	14.6
IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	16.1	21.6
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	45.7	41.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	608.6	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	243.3	265.4
IPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.5	9.3
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	9.7	3.6
IPL [Member] | IPL's electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.8	14.6
IPL [Member] | IPL's electric transmission assets sale [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	16.1	21.6
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	24.3	22.7
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	254.9	206.6
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	141.4	141.2
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	53.6	43.4
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	38.5	3.6
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 21.4	$ 18.4