Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory assets	$ 1,908.6	$ 1,783.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	987.7	955.3
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	579.5	570.2
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	92.4	73.7
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	70.6	46.9
WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	45.0	0.0
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.9	31.1
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.9	27.4
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.4	16.0
Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	14.3	16.1
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.9	47.0
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,402.6	1,357.9
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	958.2	928.0
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	298.1	287.9
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	50.8	41.4
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.2	28.0
IPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	2.8	0.4
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.9	27.4
IPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	11.0	11.5
IPL [Member] | Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	9.5	10.9
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	17.1	22.4
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	506.0	425.8
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.5	27.3
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	281.4	282.3
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.6	32.3
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	42.4	18.9
WPL [Member] | WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	45.0	0.0
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	33.1	30.7
WPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.4	4.5
WPL [Member] | Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.8	5.2
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 22.8	$ 24.6