Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,497.2	$ 681.0
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	899.4	7.7
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	410.0	411.6
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	90.4	72.0
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.0	83.5
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.0	30.8
Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.9	20.5
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	31.5	54.9
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	755.4	430.8
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	399.5	1.9
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	274.5	269.4
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	26.4	35.7
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.0	83.5
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	14.6	17.8
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	15.4	22.5
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	741.8	250.2
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	499.9	5.8
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	135.5	142.2
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	64.0	36.3
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.4	13.0
WPL [Member] | Energy efficiency cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	19.9	20.5
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 16.1	$ 32.4