Valuation And Qualifying Accounts and Reserves (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Accumulated Provision for Uncollectible Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 4.2	[1]	$ 3.2	[1]	$ 4.5	[1]
Charged to Expense	6.6	[1]	7.8	[1]	6.8	[1]
Charged to Other Accounts	1.2	[1],[2]	1.9	[1],[2]	1.8	[1],[2]
Deductions	8.0	[1],[3]	8.7	[1],[3]	9.9	[1],[3]
Ending Balance	4.0	[1]	4.2	[1]	3.2	[1]
Accumulated Provision for Uncollectible Accounts [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	0.9	[1]	0.4	[1]	2.5	[1]
Charged to Expense	6.4	[1]	7.4	[1]	5.6	[1]
Charged to Other Accounts	0	[1],[2]	0	[1],[2]	0	[1],[2]
Deductions	6.6	[1],[3]	6.9	[1],[3]	7.7	[1],[3]
Ending Balance	0.7	[1]	0.9	[1]	0.4	[1]
Accumulated Provision for Uncollectible Accounts [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	1.9		1.7		2.0
Charged to Expense	0.1		0.1		0.1
Charged to Other Accounts	1.2	[2]	1.9	[2]	1.8	[2]
Deductions	1.4	[3]	1.8	[3]	2.2	[3]
Ending Balance	1.8		1.9		1.7
Accumulated Provision for Other Reserves [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	25.9	[4]	24.0	[4]	20.9	[4]
Charged to Expense	9.6	[4]	8.8	[4]	11.5	[4]
Charged to Other Accounts	0	[2],[4]	0	[2],[4]	0	[2],[4]
Deductions	2.1	[3],[4]	6.9	[3],[4]	8.4	[3],[4]
Ending Balance	33.4	[4]	25.9	[4]	24.0	[4]
Accumulated Provision for Other Reserves [Member] | IPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	10.2	[4]	8.8	[4]	9.0	[4]
Charged to Expense	2.1	[4]	3.6	[4]	2.0	[4]
Charged to Other Accounts	0	[2],[4]	0	[2],[4]	0	[2],[4]
Deductions	0.7	[3],[4]	2.2	[3],[4]	2.2	[3],[4]
Ending Balance	11.6	[4]	10.2	[4]	8.8	[4]
Accumulated Provision for Other Reserves [Member] | WPL [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	11.7	[4]	12.8	[4]	10.7	[4]
Charged to Expense	3.1	[4]	3.7	[4]	7.5	[4]
Charged to Other Accounts	0	[2],[4]	0	[2],[4]	0	[2],[4]
Deductions	1.3	[3],[4]	4.8	[3],[4]	5.4	[3],[4]
Ending Balance	$ 13.5	[4]	$ 11.7	[4]	$ 12.8	[4]

[1] Refer to Note 4(a) of the “Combined Notes to Consolidated Financial Statements” 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 Wisconsin Power and Light Company are recorded in regulatory assets.

[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.