Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	$ (504.0)	$ (489.9)
Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0.0	0.0
Current liabilities	(2.2)	(6.5)
Pension and other benefit obligations	(350.2)	(342.1)
Net amounts recognized at December 31	(352.4)	(348.6)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	509.1	535.1
Prior service credit	(6.5)	(6.9)
Net amount recognized at December 31	502.6	528.2
Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	8.8	3.2
Current liabilities	(9.1)	(8.6)
Pension and other benefit obligations	(110.9)	(108.9)
Net amounts recognized at December 31	(111.2)	(114.3)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	47.4	52.6
Prior service credit	(1.3)	(1.5)
Net amount recognized at December 31	46.1	51.1
IPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(173.8)	(173.2)
IPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0.0	0.0
Current liabilities	(0.5)	(0.7)
Pension and other benefit obligations	(148.7)	(147.7)
Net amounts recognized at December 31	(149.2)	(148.4)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	218.9	233.6
Prior service credit	(2.1)	(2.3)
Net amount recognized at December 31	216.8	231.3
IPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	5.9	0.4
Current liabilities	(2.0)	(1.9)
Pension and other benefit obligations	(20.4)	(20.4)
Net amounts recognized at December 31	(16.5)	(21.9)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	18.7	25.4
Prior service credit	0.0	0.0
Net amount recognized at December 31	18.7	25.4
WPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(213.7)	(207.8)
WPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0.0	0.0
Current liabilities	(0.1)	(0.1)
Pension and other benefit obligations	(144.7)	(139.4)
Net amounts recognized at December 31	(144.8)	(139.5)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	224.7	236.1
Prior service credit	(1.5)	(1.4)
Net amount recognized at December 31	223.2	234.7
WPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	2.9	2.7
Current liabilities	(6.8)	(6.4)
Pension and other benefit obligations	(67.8)	(66.6)
Net amounts recognized at December 31	(71.7)	(70.3)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	23.6	21.5
Prior service credit	(1.3)	(1.5)
Net amount recognized at December 31	$ 22.3	$ 20.0