Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 628.5	$ 681.0
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	412.1	411.6
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	94.8	72.0
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	45.0	83.5
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.2	30.8
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.0	20.5
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	10.9	31.5
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	24.5	31.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	384.8	430.8
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	272.9	269.4
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.9	35.7
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	45.0	83.5
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	15.0	17.8
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.8	12.1
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	10.2	12.3
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	243.7	250.2
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	139.2	142.2
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	58.9	36.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.2	13.0
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.0	20.5
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.1	19.4
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 14.3	$ 18.8