Valuation And Qualifying Accounts and Reserves (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Provision for Uncollectible Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[1]	$ 8.7	$ 4.8	$ 5.1
Charged to Expense	[1]	15.1	17.4	8.1
Charged to Other Accounts	[1],[2]	5.4	8.8	3.0
Deductions	[1],[3]	17.2	22.3	11.4
Ending Balance	[1]	12.0	8.7	4.8
Accumulated Provision for Uncollectible Accounts [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[1]	1.1	0.6	0.4
Charged to Expense	[1]	14.9	17.2	8.1
Charged to Other Accounts	[1],[2]	0.0	0.0	0.0
Deductions	[1],[3]	14.7	16.7	7.9
Ending Balance	[1]	1.3	1.1	0.6
Accumulated Provision for Uncollectible Accounts [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance		7.1	3.7	4.2
Charged to Expense		0.2	0.1	0.0
Charged to Other Accounts	[2]	5.4	8.8	3.0
Deductions	[3]	2.0	5.5	3.5
Ending Balance		10.7	7.1	3.7
Accumulated Provision for Other Reserves [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	25.1	27.1	32.6
Charged to Expense	[4]	3.3	6.1	6.5
Charged to Other Accounts	[2],[4]	5.1	0.0	0.0
Deductions	[3],[4]	10.5	8.1	12.0
Ending Balance	[4]	23.0	25.1	27.1
Accumulated Provision for Other Reserves [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	8.7	9.4	10.6
Charged to Expense	[4]	0.3	1.0	2.1
Charged to Other Accounts	[2],[4]	0.0	0.0	0.0
Deductions	[3],[4]	1.4	1.7	3.3
Ending Balance	[4]	7.6	8.7	9.4
Accumulated Provision for Other Reserves [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	[4]	8.1	11.4	16.3
Charged to Expense	[4]	0.1	1.8	0.7
Charged to Other Accounts	[2],[4]	0.0	0.0	0.0
Deductions	[3],[4]	1.8	5.1	5.6
Ending Balance	[4]	$ 6.4	$ 8.1	$ 11.4

[1] Refer to Note 5(b) for discussion of IPL’s sales of accounts receivable program.

[2] Accumulated provision for uncollectible accounts: In accordance with its regulatory treatment, certain amounts provided by WPL are recorded in regulatory assets. WPL expenses these amounts when an uncollectible account is written-off.Accumulated provision for other reserves: In 2017, Alliant Energy recorded amounts to deferred tax liabilities related to the impacts of Tax Reform, which is discussed in Note 11.

[3] Deductions are of the nature for which the reserves were created. In the case of the accumulated provision for uncollectible accounts, deductions from this reserve are reduced by recoveries of amounts previously written off.

[4] Other reserves are largely related to injury and damage claims arising in the ordinary course of business, and the impacts of Tax Reform.