Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		$ 177.5	$ 184.5
Revisions in estimated cash flows		(5.5)	(10.1)
Liabilities settled		(8.8)	(10.4)
Liabilities incurred	[1]	26.2	7.3
Accretion expense		6.9	6.2
Balance, December 31		196.3	177.5
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		118.3	134.1
Revisions in estimated cash flows		(6.7)	(10.1)
Liabilities settled		(8.4)	(9.7)
Liabilities incurred	[1]	26.2	0.0
Accretion expense		4.5	4.0
Balance, December 31		133.9	118.3
WPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		59.2	50.4
Revisions in estimated cash flows		1.2	0.0
Liabilities settled		(0.4)	(0.7)
Liabilities incurred		0.0	7.3
Accretion expense		2.4	2.2
Balance, December 31		$ 62.4	$ 59.2

[1] In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.