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Valuation And Qualifying Accounts and Reserves (Details) - USD ($) $ in Millions | 12 Months Ended
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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.

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- Definition Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- Definition Amount of increase in the allowances and reserves, which consist of the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs, from charges to costs and expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- Definition Amount of increase in the allowances and reserves, which consist of the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs, from charges to accounts other than costs and expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- Definition Total of the deductions in a given period to allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs, representing receivables written off as uncollectible and portions of the reserves utilized, respectively. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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