Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 1,737.3	$ 1,666.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	820.6	778.2
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	542.3	548.0
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	111.6	63.8
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	110.8	109.3
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.0	45.3
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.6	25.5
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	100.4	96.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,279.0	1,231.6
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	783.1	750.5
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	274.0	274.4
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	55.4	31.6
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	76.3	72.5
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	15.1	21.8
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.6	25.5
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	51.5	55.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	458.3	435.1
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	37.5	27.7
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	268.3	273.6
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	56.2	32.2
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.5	36.8
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	12.9	23.5
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 48.9	$ 41.3