Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 737.7	$ 821.2
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	406.0	421.7
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	159.2	243.0
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	48.3	64.3
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	43.5	19.4
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	37.6	15.4
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	43.1	57.4
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	489.2	583.5
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	260.4	279.1
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	159.2	243.0
IPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.0	0.0
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.9	19.4
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	23.5	15.1
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	24.2	26.9
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	248.5	237.7
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	145.6	142.6
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	48.3	64.3
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.6	0.0
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.1	0.3
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 18.9	$ 30.5