Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	$ (489.9)	$ (451.8)
Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0.0	0.0
Other current liabilities	(6.5)	(2.6)
Pension and other benefit obligations	(342.1)	(308.7)
Net amounts recognized at December 31	(348.6)	(311.3)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	535.1	533.1
Prior service credit	(6.9)	(7.2)
Net amount recognized at December 31	528.2	525.9
Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	3.2	3.0
Other current liabilities	(8.6)	(6.2)
Pension and other benefit obligations	(108.9)	(111.3)
Net amounts recognized at December 31	(114.3)	(114.5)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	52.6	59.8
Prior service credit	(1.5)	(5.6)
Net amount recognized at December 31	51.1	54.2
IPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(173.2)	(160.2)
IPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0.0	0.0
Other current liabilities	(0.7)	(0.8)
Pension and other benefit obligations	(147.7)	(132.6)
Net amounts recognized at December 31	(148.4)	(133.4)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	233.6	235.5
Prior service credit	(2.3)	(2.5)
Net amount recognized at December 31	231.3	233.0
IPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0.4	0.0
Other current liabilities	(1.9)	0.0
Pension and other benefit obligations	(20.4)	(22.1)
Net amounts recognized at December 31	(21.9)	(22.1)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	25.4	29.8
Prior service credit	0.0	(2.7)
Net amount recognized at December 31	25.4	27.1
WPL [Member]
Amounts recognized on the balance sheets consist of:
Pension and other benefit obligations	(207.8)	(188.7)
WPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	0.0	0.0
Other current liabilities	(0.1)	(0.1)
Pension and other benefit obligations	(139.4)	(119.0)
Net amounts recognized at December 31	(139.5)	(119.1)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	236.1	232.1
Prior service credit	(1.4)	(1.2)
Net amount recognized at December 31	234.7	230.9
WPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the balance sheets consist of:
Non-current assets	2.7	3.0
Other current liabilities	(6.4)	(6.0)
Pension and other benefit obligations	(66.6)	(68.0)
Net amounts recognized at December 31	(70.3)	(71.0)
Amounts recognized in Regulatory Assets and AOCL consist of:
Net actuarial loss	21.5	23.3
Prior service credit	(1.5)	(2.4)
Net amount recognized at December 31	$ 20.0	$ 20.9