Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax liabilities, property	$ 1,975.5	$ 1,852.7
Deferred tax liabilities, other	80.5	75.9
Total deferred tax liabilities	2,158.4	2,015.0
Deferred tax assets, federal credit carryforwards	299.1	260.7
Deferred tax assets, net operating losses carryforwards - federal	158.6	174.4
Deferred tax assets, net operating losses carryforwards - state	47.0	41.3
Deferred tax assets, other	59.8	69.2
Subtotal deferred tax assets	564.5	545.6
Deferred tax assets, valuation allowances	(9.2)	(9.0)
Total deferred tax assets	555.3	536.6
Total deferred tax liabilities, net	1,603.1	1,478.4
IPL [Member]
Deferred tax liabilities, property	1,158.4	1,102.6
Deferred tax liabilities, other	69.4	63.4
Total deferred tax liabilities	1,227.8	1,166.0
Deferred tax assets, federal credit carryforwards	133.2	113.1
Deferred tax assets, net operating losses carryforwards - federal	114.1	107.4
Deferred tax assets, net operating losses carryforwards - state	0.9	0.9
Deferred tax assets, other	22.8	34.5
Subtotal deferred tax assets	271.0	255.9
Deferred tax assets, valuation allowances	(0.5)	(0.6)
Total deferred tax assets	270.5	255.3
Total deferred tax liabilities, net	957.3	910.7
WPL [Member]
Deferred tax liabilities, property	735.2	674.2
Deferred tax liabilities, other	34.4	36.5
Total deferred tax liabilities	769.6	710.7
Deferred tax assets, federal credit carryforwards	147.4	131.0
Deferred tax assets, net operating losses carryforwards - federal	26.4	43.7
Deferred tax assets, net operating losses carryforwards - state	0.5	0.2
Deferred tax assets, other	14.1	14.7
Subtotal deferred tax assets	188.4	189.6
Deferred tax assets, valuation allowances	(0.8)	(1.3)
Total deferred tax assets	187.6	188.3
Total deferred tax liabilities, net	582.0	522.4
ATC Holdings [Member]
Deferred tax liabilities, ATC Holdings	$ 102.4	$ 86.4