Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 681.0	$ 737.7
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	411.6	406.0
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	83.5	159.2
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	72.0	43.5
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	31.5	8.5
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	30.8	37.6
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.5	48.3
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	31.1	34.6
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	430.8	489.2
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	269.4	260.4
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	83.5	159.2
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	35.7	21.9
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	12.1	6.7
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	17.8	23.5
IPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.0	0.0
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	12.3	17.5
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	250.2	248.5
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	142.2	145.6
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	36.3	21.6
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.4	1.8
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	13.0	14.1
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.5	48.3
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 18.8	$ 17.1