Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	$ 196.3
Revisions in estimated cash flows	0.9
Liabilities settled	(3.5)
Liabilities incurred	27.6	[1]
Accretion expense	3.5
Balance, June 30	224.8
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	133.9
Revisions in estimated cash flows	0.0
Liabilities settled	(2.8)
Liabilities incurred	27.6	[1]
Accretion expense	2.5
Balance, June 30	$ 161.2

[1] During the six months ended June 30, 2020, Alliant Energy and IPL recognized additional asset retirement obligations related to IPL’s newly constructed Whispering Willow North and Golden Plains wind sites. The increases in asset retirement obligations resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.