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Valuation and Qualifying Accounts And Reserves | 12 Months Ended
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Dec. 31, 2017
Valuation and Qualifying Accounts Disclosure [Line Items] |
Valuation and Qualifying Accounts And Reserves | SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES
|  |  |  | Additions |  |
| Balance, | Charged to | Charged to Other |  | Balance,
Description | January 1 | Expense | Accounts (a) | Deductions (b) | December 31
| (in millions)
Valuation and Qualifying Accounts Which are Deducted in the Balance Sheet From the Assets to Which They Apply:
| Accumulated Provision for Uncollectible Accounts: |  |  |  |
|  | Alliant Energy (c) |  |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.7 |  |  | $15.1 |  |  | $5.4 |  |  | $17.2 |  |  | $12.0 |
|  |  | Year ended December 31, 2016 | 4.8 |  | 17.4 |  | 8.8 |  | 22.3 |  | 8.7 |
|  |  | Year ended December 31, 2015 | 5.1 |  | 8.1 |  | 3.0 |  | 11.4 |  | 4.8 |
|  | IPL (c) |  |  |  |
|  |  | Year ended December 31, 2017 |  | $1.1 |  |  | $14.9 |  |  | $— |  |  | $14.7 |  |  | $1.3 |
|  |  | Year ended December 31, 2016 | 0.6 |  | 17.2 |  | — |  | 16.7 |  | 1.1 |
|  |  | Year ended December 31, 2015 | 0.4 |  | 8.1 |  | — |  | 7.9 |  | 0.6 |
|  | WPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $7.1 |  |  | $0.2 |  |  | $5.4 |  |  | $2.0 |  |  | $10.7 |
|  |  | Year ended December 31, 2016 | 3.7 |  | 0.1 |  | 8.8 |  | 5.5 |  | 7.1 |
|  |  | Year ended December 31, 2015 | 4.2 |  | — |  | 3.0 |  | 3.5 |  | 3.7 |
Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. Other Reserves:
| Accumulated Provision for Other Reserves (d):
|  | Alliant Energy |  |  |  |
|  |  | Year ended December 31, 2017 |  | $25.1 |  |  | $3.3 |  |  | $5.1 |  |  | $10.5 |  |  | $23.0 |
|  |  | Year ended December 31, 2016 | 27.1 |  | 6.1 |  | — |  | 8.1 |  | 25.1 |
|  |  | Year ended December 31, 2015 | 32.6 |  | 6.5 |  | — |  | 12.0 |  | 27.1 |
|  | IPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.7 |  |  | $0.3 |  |  | $— |  |  | $1.4 |  |  | $7.6 |
|  |  | Year ended December 31, 2016 | 9.4 |  | 1.0 |  | — |  | 1.7 |  | 8.7 |
|  |  | Year ended December 31, 2015 | 10.6 |  | 2.1 |  | — |  | 3.3 |  | 9.4 |
|  | WPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.1 |  |  | $0.1 |  |  | $— |  |  | $1.8 |  |  | $6.4 |
|  |  | Year ended December 31, 2016 | 11.4 |  | 1.8 |  | — |  | 5.1 |  | 8.1 |
|  |  | Year ended December 31, 2015 | 16.3 |  | 0.7 |  | — |  | 5.6 |  | 11.4 |
(a) | 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.
(b) | 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.
(c) | Refer to Note 5(b) for discussion of IPL’s sales of accounts receivable program.
(d) | Other reserves are largely related to injury and damage claims arising in the ordinary course of business, and the impacts of Tax Reform.
IPL [Member] |
Valuation and Qualifying Accounts Disclosure [Line Items] |
Valuation and Qualifying Accounts And Reserves | SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES
|  |  |  | Additions |  |
| Balance, | Charged to | Charged to Other |  | Balance,
Description | January 1 | Expense | Accounts (a) | Deductions (b) | December 31
| (in millions)
Valuation and Qualifying Accounts Which are Deducted in the Balance Sheet From the Assets to Which They Apply:
| Accumulated Provision for Uncollectible Accounts: |  |  |  |
|  | Alliant Energy (c) |  |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.7 |  |  | $15.1 |  |  | $5.4 |  |  | $17.2 |  |  | $12.0 |
|  |  | Year ended December 31, 2016 | 4.8 |  | 17.4 |  | 8.8 |  | 22.3 |  | 8.7 |
|  |  | Year ended December 31, 2015 | 5.1 |  | 8.1 |  | 3.0 |  | 11.4 |  | 4.8 |
|  | IPL (c) |  |  |  |
|  |  | Year ended December 31, 2017 |  | $1.1 |  |  | $14.9 |  |  | $— |  |  | $14.7 |  |  | $1.3 |
|  |  | Year ended December 31, 2016 | 0.6 |  | 17.2 |  | — |  | 16.7 |  | 1.1 |
|  |  | Year ended December 31, 2015 | 0.4 |  | 8.1 |  | — |  | 7.9 |  | 0.6 |
|  | WPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $7.1 |  |  | $0.2 |  |  | $5.4 |  |  | $2.0 |  |  | $10.7 |
|  |  | Year ended December 31, 2016 | 3.7 |  | 0.1 |  | 8.8 |  | 5.5 |  | 7.1 |
|  |  | Year ended December 31, 2015 | 4.2 |  | — |  | 3.0 |  | 3.5 |  | 3.7 |
Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. Other Reserves:
| Accumulated Provision for Other Reserves (d):
|  | Alliant Energy |  |  |  |
|  |  | Year ended December 31, 2017 |  | $25.1 |  |  | $3.3 |  |  | $5.1 |  |  | $10.5 |  |  | $23.0 |
|  |  | Year ended December 31, 2016 | 27.1 |  | 6.1 |  | — |  | 8.1 |  | 25.1 |
|  |  | Year ended December 31, 2015 | 32.6 |  | 6.5 |  | — |  | 12.0 |  | 27.1 |
|  | IPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.7 |  |  | $0.3 |  |  | $— |  |  | $1.4 |  |  | $7.6 |
|  |  | Year ended December 31, 2016 | 9.4 |  | 1.0 |  | — |  | 1.7 |  | 8.7 |
|  |  | Year ended December 31, 2015 | 10.6 |  | 2.1 |  | — |  | 3.3 |  | 9.4 |
|  | WPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.1 |  |  | $0.1 |  |  | $— |  |  | $1.8 |  |  | $6.4 |
|  |  | Year ended December 31, 2016 | 11.4 |  | 1.8 |  | — |  | 5.1 |  | 8.1 |
|  |  | Year ended December 31, 2015 | 16.3 |  | 0.7 |  | — |  | 5.6 |  | 11.4 |
(a) | 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.
(b) | 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.
(c) | Refer to Note 5(b) for discussion of IPL’s sales of accounts receivable program.
(d) | Other reserves are largely related to injury and damage claims arising in the ordinary course of business, and the impacts of Tax Reform.
WPL [Member] |
Valuation and Qualifying Accounts Disclosure [Line Items] |
Valuation and Qualifying Accounts And Reserves | SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES
|  |  |  | Additions |  |
| Balance, | Charged to | Charged to Other |  | Balance,
Description | January 1 | Expense | Accounts (a) | Deductions (b) | December 31
| (in millions)
Valuation and Qualifying Accounts Which are Deducted in the Balance Sheet From the Assets to Which They Apply:
| Accumulated Provision for Uncollectible Accounts: |  |  |  |
|  | Alliant Energy (c) |  |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.7 |  |  | $15.1 |  |  | $5.4 |  |  | $17.2 |  |  | $12.0 |
|  |  | Year ended December 31, 2016 | 4.8 |  | 17.4 |  | 8.8 |  | 22.3 |  | 8.7 |
|  |  | Year ended December 31, 2015 | 5.1 |  | 8.1 |  | 3.0 |  | 11.4 |  | 4.8 |
|  | IPL (c) |  |  |  |
|  |  | Year ended December 31, 2017 |  | $1.1 |  |  | $14.9 |  |  | $— |  |  | $14.7 |  |  | $1.3 |
|  |  | Year ended December 31, 2016 | 0.6 |  | 17.2 |  | — |  | 16.7 |  | 1.1 |
|  |  | Year ended December 31, 2015 | 0.4 |  | 8.1 |  | — |  | 7.9 |  | 0.6 |
|  | WPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $7.1 |  |  | $0.2 |  |  | $5.4 |  |  | $2.0 |  |  | $10.7 |
|  |  | Year ended December 31, 2016 | 3.7 |  | 0.1 |  | 8.8 |  | 5.5 |  | 7.1 |
|  |  | Year ended December 31, 2015 | 4.2 |  | — |  | 3.0 |  | 3.5 |  | 3.7 |
Note: The above provisions relate to various customer, notes and other receivable balances included in various line items on the respective balance sheets. Other Reserves:
| Accumulated Provision for Other Reserves (d):
|  | Alliant Energy |  |  |  |
|  |  | Year ended December 31, 2017 |  | $25.1 |  |  | $3.3 |  |  | $5.1 |  |  | $10.5 |  |  | $23.0 |
|  |  | Year ended December 31, 2016 | 27.1 |  | 6.1 |  | — |  | 8.1 |  | 25.1 |
|  |  | Year ended December 31, 2015 | 32.6 |  | 6.5 |  | — |  | 12.0 |  | 27.1 |
|  | IPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.7 |  |  | $0.3 |  |  | $— |  |  | $1.4 |  |  | $7.6 |
|  |  | Year ended December 31, 2016 | 9.4 |  | 1.0 |  | — |  | 1.7 |  | 8.7 |
|  |  | Year ended December 31, 2015 | 10.6 |  | 2.1 |  | — |  | 3.3 |  | 9.4 |
|  | WPL |  |  |  |
|  |  | Year ended December 31, 2017 |  | $8.1 |  |  | $0.1 |  |  | $— |  |  | $1.8 |  |  | $6.4 |
|  |  | Year ended December 31, 2016 | 11.4 |  | 1.8 |  | — |  | 5.1 |  | 8.1 |
|  |  | Year ended December 31, 2015 | 16.3 |  | 0.7 |  | — |  | 5.6 |  | 11.4 |
(a) | 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.
(b) | 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.
(c) | Refer to Note 5(b) for discussion of IPL’s sales of accounts receivable program.
(d) | 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 The entire disclosure for any allowance and reserve accounts (their beginning and ending balances, as well as a reconciliation by type of activity during the period). Alternatively, disclosure of the required information may be within the footnotes to the financial statements or a supplemental schedule to the financial statements. + 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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