Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 696.6	$ 681.0
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	413.0	411.6
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	128.3	72.0
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	64.7	83.5
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	32.9	30.8
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.6	20.5
Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	9.0	31.5
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	28.1	31.1
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	448.3	430.8
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	271.1	269.4
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	75.9	35.7
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	64.7	83.5
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.1	17.8
IPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	0.0	0.0
IPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5.1	12.1
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11.4	12.3
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	248.3	250.2
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	141.9	142.2
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	52.4	36.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	12.8	13.0
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	20.6	20.5
WPL [Member] | Derivatives [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	3.9	19.4
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 16.7	$ 18.8