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Property, Plant and Equipment (Tables) | 12 Months Ended
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Dec. 31, 2017
Property, Plant and Equipment [Line Items] |
Property, Plant and Equipment | At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Utility: |  |  |  |  |  |  |  |  |  |  |
Electric plant: |  |  |  |  |  |  |  |  |  |  |
Generation in service |  | $6,655.3 |  |  |  | $5,866.9 |  |  |  | $3,715.9 |  |  |  | $2,916.8 |  |  |  | $2,939.4 |  |  |  | $2,950.1 |
Distribution in service | 5,123.5 |  |  | 4,739.2 |  |  | 2,820.9 |  |  | 2,589.3 |  |  | 2,302.6 |  |  | 2,149.9 |
Other in service | 425.1 |  |  | 329.1 |  |  | 282.3 |  |  | 223.5 |  |  | 142.8 |  |  | 105.6 |
Anticipated to be retired early (a)(b) | 93.0 |  |  | 108.3 |  |  | — |  |  | 108.3 |  |  | 93.0 |  |  | — |
Total electric plant | 12,296.9 |  |  | 11,043.5 |  |  | 6,819.1 |  |  | 5,837.9 |  |  | 5,477.8 |  |  | 5,205.6 |
Gas plant in service | 1,244.0 |  |  | 1,107.6 |  |  | 654.8 |  |  | 556.7 |  |  | 589.2 |  |  | 550.9 |
Other plant in service | 571.9 |  |  | 549.3 |  |  | 333.4 |  |  | 313.0 |  |  | 238.5 |  |  | 236.3 |
Accumulated depreciation | (4,283.1 | ) |  | (4,135.7 | ) |  | (2,311.0 | ) |  | (2,258.3 | ) |  | (1,972.1 | ) |  | (1,877.4 | )
Net plant | 9,829.7 |  |  | 8,564.7 |  |  | 5,496.3 |  |  | 4,449.3 |  |  | 4,333.4 |  |  | 4,115.4 |
Leased Sheboygan Falls Energy Facility, net (c) | — |  |  | — |  |  | — |  |  | — |  |  | 46.2 |  |  | 52.4 |
Construction work in progress | 962.2 |  |  | 1,226.8 |  |  | 424.4 |  |  | 968.1 |  |  | 537.8 |  |  | 258.7 |
Other, net | 6.0 |  |  | 18.4 |  |  | 5.5 |  |  | 18.2 |  |  | 0.5 |  |  | 0.2 |
Total utility | 10,797.9 |  |  | 9,809.9 |  |  | 5,926.2 |  |  | 5,435.6 |  |  | 4,917.9 |  |  | 4,426.7 |
Non-utility and other: |  |  |  |  |  |  |  |  |  |  |
Non-utility Generation, net (d) | 90.9 |  |  | 135.0 |  |  | — |  |  | — |  |  | — |  |  | — |
Corporate Services and other, net (e) | 345.7 |  |  | 334.3 |  |  | — |  |  | — |  |  | — |  |  | — |
Total non-utility and other | 436.6 |  |  | 469.3 |  |  | — |  |  | — |  |  | — |  |  | — |
Total property, plant and equipment |  | $11,234.5 |  |  |  | $10,279.2 |  |  |  | $5,926.2 |  |  |  | $5,435.6 |  |  |  | $4,917.9 |  |  |  | $4,426.7 |
(a) | In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3.
(b) | In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10-year period beginning the later of the retirement date of the EGU or January 1, 2019.
(c) | Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016, respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment.
(d) | Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016, respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order.
(e) | Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016, respectively.
Assets Purchased and Liabilities Assumed | As of the closing date, the estimated fair values of the assets purchased and liabilities assumed by IPL were as follows (in millions):
Electric plant in service |  | $40 |
Current assets | 2 |
Total assets acquired | 42 |
Other liabilities | 10 |
Net assets acquired |  | $32 |
Allowance For Funds Used During Construction | The amount of AFUDC generated by equity and debt components was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Equity |  | $33.6 |  |  |  | $42.3 |  |  |  | $24.4 |  |  |  | $21.1 |  |  |  | $35.2 |  |  |  | $18.6 |  |  |  | $12.5 |  |  |  | $7.1 |  |  |  | $5.8 |
Debt | 16.1 |  |  | 20.2 |  |  | 12.5 |  |  | 10.3 |  |  | 16.8 |  |  | 9.6 |  |  | 5.8 |  |  | 3.4 |  |  | 2.9 |
|  | $49.7 |  |  |  | $62.5 |  |  |  | $36.9 |  |  |  | $31.4 |  |  |  | $52.0 |  |  |  | $28.2 |  |  |  | $18.3 |  |  |  | $10.5 |  |  |  | $8.7 |
IPL [Member] |
Property, Plant and Equipment [Line Items] |
Property, Plant and Equipment | At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Utility: |  |  |  |  |  |  |  |  |  |  |
Electric plant: |  |  |  |  |  |  |  |  |  |  |
Generation in service |  | $6,655.3 |  |  |  | $5,866.9 |  |  |  | $3,715.9 |  |  |  | $2,916.8 |  |  |  | $2,939.4 |  |  |  | $2,950.1 |
Distribution in service | 5,123.5 |  |  | 4,739.2 |  |  | 2,820.9 |  |  | 2,589.3 |  |  | 2,302.6 |  |  | 2,149.9 |
Other in service | 425.1 |  |  | 329.1 |  |  | 282.3 |  |  | 223.5 |  |  | 142.8 |  |  | 105.6 |
Anticipated to be retired early (a)(b) | 93.0 |  |  | 108.3 |  |  | — |  |  | 108.3 |  |  | 93.0 |  |  | — |
Total electric plant | 12,296.9 |  |  | 11,043.5 |  |  | 6,819.1 |  |  | 5,837.9 |  |  | 5,477.8 |  |  | 5,205.6 |
Gas plant in service | 1,244.0 |  |  | 1,107.6 |  |  | 654.8 |  |  | 556.7 |  |  | 589.2 |  |  | 550.9 |
Other plant in service | 571.9 |  |  | 549.3 |  |  | 333.4 |  |  | 313.0 |  |  | 238.5 |  |  | 236.3 |
Accumulated depreciation | (4,283.1 | ) |  | (4,135.7 | ) |  | (2,311.0 | ) |  | (2,258.3 | ) |  | (1,972.1 | ) |  | (1,877.4 | )
Net plant | 9,829.7 |  |  | 8,564.7 |  |  | 5,496.3 |  |  | 4,449.3 |  |  | 4,333.4 |  |  | 4,115.4 |
Leased Sheboygan Falls Energy Facility, net (c) | — |  |  | — |  |  | — |  |  | — |  |  | 46.2 |  |  | 52.4 |
Construction work in progress | 962.2 |  |  | 1,226.8 |  |  | 424.4 |  |  | 968.1 |  |  | 537.8 |  |  | 258.7 |
Other, net | 6.0 |  |  | 18.4 |  |  | 5.5 |  |  | 18.2 |  |  | 0.5 |  |  | 0.2 |
Total utility | 10,797.9 |  |  | 9,809.9 |  |  | 5,926.2 |  |  | 5,435.6 |  |  | 4,917.9 |  |  | 4,426.7 |
Non-utility and other: |  |  |  |  |  |  |  |  |  |  |
Non-utility Generation, net (d) | 90.9 |  |  | 135.0 |  |  | — |  |  | — |  |  | — |  |  | — |
Corporate Services and other, net (e) | 345.7 |  |  | 334.3 |  |  | — |  |  | — |  |  | — |  |  | — |
Total non-utility and other | 436.6 |  |  | 469.3 |  |  | — |  |  | — |  |  | — |  |  | — |
Total property, plant and equipment |  | $11,234.5 |  |  |  | $10,279.2 |  |  |  | $5,926.2 |  |  |  | $5,435.6 |  |  |  | $4,917.9 |  |  |  | $4,426.7 |
(a) | In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3.
(b) | In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10-year period beginning the later of the retirement date of the EGU or January 1, 2019.
(c) | Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016, respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment.
(d) | Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016, respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order.
(e) | Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016, respectively.
Assets Purchased and Liabilities Assumed | As of the closing date, the estimated fair values of the assets purchased and liabilities assumed by IPL were as follows (in millions):
Electric plant in service |  | $40 |
Current assets | 2 |
Total assets acquired | 42 |
Other liabilities | 10 |
Net assets acquired |  | $32 |
Allowance For Funds Used During Construction | The amount of AFUDC generated by equity and debt components was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Equity |  | $33.6 |  |  |  | $42.3 |  |  |  | $24.4 |  |  |  | $21.1 |  |  |  | $35.2 |  |  |  | $18.6 |  |  |  | $12.5 |  |  |  | $7.1 |  |  |  | $5.8 |
Debt | 16.1 |  |  | 20.2 |  |  | 12.5 |  |  | 10.3 |  |  | 16.8 |  |  | 9.6 |  |  | 5.8 |  |  | 3.4 |  |  | 2.9 |
|  | $49.7 |  |  |  | $62.5 |  |  |  | $36.9 |  |  |  | $31.4 |  |  |  | $52.0 |  |  |  | $28.2 |  |  |  | $18.3 |  |  |  | $10.5 |  |  |  | $8.7 |
WPL [Member] |
Property, Plant and Equipment [Line Items] |
Property, Plant and Equipment | At December 31, details of property, plant and equipment on the balance sheets were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Utility: |  |  |  |  |  |  |  |  |  |  |
Electric plant: |  |  |  |  |  |  |  |  |  |  |
Generation in service |  | $6,655.3 |  |  |  | $5,866.9 |  |  |  | $3,715.9 |  |  |  | $2,916.8 |  |  |  | $2,939.4 |  |  |  | $2,950.1 |
Distribution in service | 5,123.5 |  |  | 4,739.2 |  |  | 2,820.9 |  |  | 2,589.3 |  |  | 2,302.6 |  |  | 2,149.9 |
Other in service | 425.1 |  |  | 329.1 |  |  | 282.3 |  |  | 223.5 |  |  | 142.8 |  |  | 105.6 |
Anticipated to be retired early (a)(b) | 93.0 |  |  | 108.3 |  |  | — |  |  | 108.3 |  |  | 93.0 |  |  | — |
Total electric plant | 12,296.9 |  |  | 11,043.5 |  |  | 6,819.1 |  |  | 5,837.9 |  |  | 5,477.8 |  |  | 5,205.6 |
Gas plant in service | 1,244.0 |  |  | 1,107.6 |  |  | 654.8 |  |  | 556.7 |  |  | 589.2 |  |  | 550.9 |
Other plant in service | 571.9 |  |  | 549.3 |  |  | 333.4 |  |  | 313.0 |  |  | 238.5 |  |  | 236.3 |
Accumulated depreciation | (4,283.1 | ) |  | (4,135.7 | ) |  | (2,311.0 | ) |  | (2,258.3 | ) |  | (1,972.1 | ) |  | (1,877.4 | )
Net plant | 9,829.7 |  |  | 8,564.7 |  |  | 5,496.3 |  |  | 4,449.3 |  |  | 4,333.4 |  |  | 4,115.4 |
Leased Sheboygan Falls Energy Facility, net (c) | — |  |  | — |  |  | — |  |  | — |  |  | 46.2 |  |  | 52.4 |
Construction work in progress | 962.2 |  |  | 1,226.8 |  |  | 424.4 |  |  | 968.1 |  |  | 537.8 |  |  | 258.7 |
Other, net | 6.0 |  |  | 18.4 |  |  | 5.5 |  |  | 18.2 |  |  | 0.5 |  |  | 0.2 |
Total utility | 10,797.9 |  |  | 9,809.9 |  |  | 5,926.2 |  |  | 5,435.6 |  |  | 4,917.9 |  |  | 4,426.7 |
Non-utility and other: |  |  |  |  |  |  |  |  |  |  |
Non-utility Generation, net (d) | 90.9 |  |  | 135.0 |  |  | — |  |  | — |  |  | — |  |  | — |
Corporate Services and other, net (e) | 345.7 |  |  | 334.3 |  |  | — |  |  | — |  |  | — |  |  | — |
Total non-utility and other | 436.6 |  |  | 469.3 |  |  | — |  |  | — |  |  | — |  |  | — |
Total property, plant and equipment |  | $11,234.5 |  |  |  | $10,279.2 |  |  |  | $5,926.2 |  |  |  | $5,435.6 |  |  |  | $4,917.9 |  |  |  | $4,426.7 |
(a) | In 2017, IPL retired Sutherland Unit 3 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. Refer to Note 2 for further discussion, including recovery of the remaining net book value of Sutherland Unit 3.
(b) | In 2017, WPL received approval from MISO to retire Edgewater Unit 4 and currently anticipates retiring this EGU by September 30, 2018. In 2016, the PSCW authorized WPL to recover the remaining net book value of Edgewater Unit 4 over a 10-year period beginning the later of the retirement date of the EGU or January 1, 2019.
(c) | Less accumulated amortization of $77.6 million and $71.4 million for WPL as of December 31, 2017 and 2016, respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment.
(d) | Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016, respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order.
(e) | Less accumulated depreciation of $285.6 million and $272.0 million for Alliant Energy as of December 31, 2017 and 2016, respectively.
Allowance For Funds Used During Construction | The amount of AFUDC generated by equity and debt components was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Equity |  | $33.6 |  |  |  | $42.3 |  |  |  | $24.4 |  |  |  | $21.1 |  |  |  | $35.2 |  |  |  | $18.6 |  |  |  | $12.5 |  |  |  | $7.1 |  |  |  | $5.8 |
Debt | 16.1 |  |  | 20.2 |  |  | 12.5 |  |  | 10.3 |  |  | 16.8 |  |  | 9.6 |  |  | 5.8 |  |  | 3.4 |  |  | 2.9 |
|  | $49.7 |  |  |  | $62.5 |  |  |  | $36.9 |  |  |  | $31.4 |  |  |  | $52.0 |  |  |  | $28.2 |  |  |  | $18.3 |  |  |  | $10.5 |  |  |  | $8.7 |

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