Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred tax liabilities, property	$ 2,919.0	$ 2,762.9
Deferred tax liabilities, other	95.3	157.3
Total deferred tax liabilities	3,167.4	3,058.3
Deferred tax assets, federal credit carryforwards	268.4	236.4
Deferred tax assets, net operating losses carryforwards - federal	173.3	250.9
Deferred tax assets, net operating losses carryforwards - state	32.9	38.3
Deferred tax assets, other	87.9	85.4
Total deferred tax assets	597.2	677.1
Total deferred tax liabilities, net	2,570.2	2,381.2
IPL [Member]
Deferred tax liabilities, property	1,677.0	1,587.8
Deferred tax liabilities, other	71.4	87.8
Total deferred tax liabilities	1,748.4	1,675.6
Deferred tax assets, federal credit carryforwards	95.9	81.7
Deferred tax assets, net operating losses carryforwards - federal	69.6	113.1
Deferred tax assets, net operating losses carryforwards - state	0.6	1.1
Deferred tax assets, other	35.8	35.6
Total deferred tax assets	236.6	297.6
Total deferred tax liabilities, net	1,511.8	1,378.0
WPL [Member]
Deferred tax liabilities, property	1,124.5	1,027.0
Deferred tax liabilities, other	59.1	67.9
Total deferred tax liabilities	1,183.6	1,233.8
Deferred tax assets, federal credit carryforwards	112.9	95.5
Deferred tax assets, net operating losses carryforwards - federal	75.4	105.1
Deferred tax assets, net operating losses carryforwards - state	0.1	3.6
Deferred tax assets, other	23.6	24.2
Total deferred tax assets	212.0	228.4
Total deferred tax liabilities, net	971.6	1,005.4
ATC [Member]
Deferred tax liabilities, investment in ATC	153.1	138.1
ATC [Member] | WPL [Member]
Deferred tax liabilities, investment in ATC	0.0	138.9
IPL's tax benefit riders [Member]
Deferred tax assets, regulatory liability - IPL's tax benefit riders	34.7	66.1
IPL's tax benefit riders [Member] | IPL [Member]
Deferred tax assets, regulatory liability - IPL's tax benefit riders	$ 34.7	$ 66.1