Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities, property	$ 1,852.7	$ 2,919.0
Deferred tax liabilities, other	75.9	95.1
Total deferred tax liabilities	2,015.0	3,167.2
Deferred tax assets, federal credit carryforwards	260.7	268.4
Deferred tax assets, net operating losses carryforwards - federal	174.4	173.3
Deferred tax assets, net operating losses carryforwards - state	41.3	32.9
Deferred tax assets, other	61.8	87.9
Subtotal deferred tax assets	545.6	597.2
Deferred tax assets, valuation allowance	(9.0)	(0.2)
Total deferred tax assets	536.6	597.0
Total deferred tax liabilities, net	1,478.4	2,570.2
IPL [Member]
Deferred tax liabilities, property	1,102.6	1,677.0
Deferred tax liabilities, other	63.4	71.4
Total deferred tax liabilities	1,166.0	1,748.4
Deferred tax assets, federal credit carryforwards	113.1	95.9
Deferred tax assets, net operating losses carryforwards - federal	107.4	69.6
Deferred tax assets, net operating losses carryforwards - state	0.9	0.6
Deferred tax assets, other	27.1	35.8
Subtotal deferred tax assets	255.9	236.6
Deferred tax assets, valuation allowance	(0.6)	0.0
Total deferred tax assets	255.3	236.6
Total deferred tax liabilities, net	910.7	1,511.8
WPL [Member]
Deferred tax liabilities, property	674.2	1,124.5
Deferred tax liabilities, other	36.5	58.8
Total deferred tax liabilities	710.7	1,183.3
Deferred tax assets, federal credit carryforwards	131.0	112.9
Deferred tax assets, net operating losses carryforwards - federal	43.7	75.4
Deferred tax assets, net operating losses carryforwards - state	0.2	0.1
Deferred tax assets, other	14.7	23.6
Subtotal deferred tax assets	189.6	212.0
Deferred tax assets, valuation allowance	(1.3)	(0.3)
Total deferred tax assets	188.3	211.7
Total deferred tax liabilities, net	522.4	971.6
ATC Investment [Member]
Deferred tax liabilities, ATC Investment	86.4	153.1
IPL's tax benefit riders [Member]
Deferred tax assets, regulatory liability - IPL's tax benefit riders	7.4	34.7
IPL's tax benefit riders [Member] | IPL [Member]
Deferred tax assets, regulatory liability - IPL's tax benefit riders	$ 7.4	$ 34.7