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Regulatory Matters | 6 Months Ended
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Jun. 30, 2017
Public Utilities, General Disclosures [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets and Regulatory Liabilities - Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016
Tax-related |  | $1,100.7 |  |  |  | $1,055.6 |  |  |  | $1,063.3 |  |  |  | $1,022.4 |  |  |  | $37.4 |  |  |  | $33.2 |
Pension and OPEB costs | 558.3 |  |  | 578.7 |  |  | 284.2 |  |  | 294.0 |  |  | 274.1 |  |  | 284.7 |
Asset retirement obligations | 108.7 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.2 |  |  | 41.6 |
EGUs retired early | 78.2 |  |  | 41.4 |  |  | 40.3 |  |  | — |  |  | 37.9 |  |  | 41.4 |
Derivatives | 47.9 |  |  | 30.7 |  |  | 18.1 |  |  | 10.0 |  |  | 29.8 |  |  | 20.7 |
Emission allowances | 25.9 |  |  | 26.2 |  |  | 25.9 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.3 |  |  | 76.6 |  |  | 52.3 |  |  | 41.9 |  |  | 44.0 |  |  | 34.7 |
|  | $2,016.0 |  |  |  | $1,915.1 |  |  |  | $1,556.6 |  |  |  | $1,458.8 |  |  |  | $459.4 |  |  |  | $456.3 |
Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016
Cost of removal obligations |  | $415.3 |  |  |  | $411.6 |  |  |  | $273.0 |  |  |  | $269.4 |  |  |  | $142.3 |  |  |  | $142.2 |
Electric transmission cost recovery | 118.5 |  |  | 72.0 |  |  | 61.7 |  |  | 35.7 |  |  | 56.8 |  |  | 36.3 |
IPL’s tax benefit riders | 47.2 |  |  | 83.5 |  |  | 47.2 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 27.5 |  |  | 30.8 |  |  | 15.9 |  |  | 17.8 |  |  | 11.6 |  |  | 13.0 |
Energy efficiency cost recovery | 19.6 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.6 |  |  | 20.5 |
Derivatives | 11.4 |  |  | 31.5 |  |  | 7.1 |  |  | 12.1 |  |  | 4.3 |  |  | 19.4 |
Other | 26.3 |  |  | 31.1 |  |  | 10.8 |  |  | 12.3 |  |  | 15.5 |  |  | 18.8 |
|  | $665.8 |  |  |  | $681.0 |  |  |  | $415.7 |  |  |  | $430.8 |  |  |  | $250.1 |  |  |  | $250.2 |
Tax-related - Alliant Energy’s and IPL’s tax-related regulatory assets are generally impacted by certain property-related differences at IPL for which deferred tax is not recorded in the income statement pursuant to Iowa rate-making principles. Deferred tax amounts for such property-related differences at IPL are recorded to regulatory assets, along with the necessary revenue requirement tax gross-ups. During the six months ended June 30, 2017, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL is currently earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL has requested continued recovery of the remaining net book value of these EGUs from both its retail and wholesale customers over a 10-year period from the IUB and FERC, with decisions currently expected in 2018 and 2017, respectively. Derivatives - Refer to Note 11 for discussion of derivative assets and derivative liabilities. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC LLC to determine electric transmission costs billed to utilities, including IPL and WPL. In September 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32%, excluding any incentive adders granted by FERC, effective September 28, 2016, and for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In the first half of 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million, $39 million and $11 million, respectively, after final true-ups. IPL and WPL each recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC is currently being refunded to its retail customers in 2017, beginning May 2017. WPL’s retail portion of the refund from ATC LLC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in the first quarter of 2017. IPL’s tax benefit riders - IPL’s tax benefit riders utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures and cost of removal expenditures. For the six months ended June 30, 2017, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $36 million as follows (in millions):
Electric tax benefit rider credits |  | $33 |
Gas tax benefit rider credits | 3 |
|  | $36 |
Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers by $176 million, or approximately 12%. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million, or approximately 7%, on an annual basis, was implemented effective April 13, 2017, without regulatory review, and will be subject to refund pending determination of final rates. Tax benefit rider credits and MISO transmission owner return on equity refunds are expected to reduce the effect of the rate increase on customer bills in 2017 and 2018. IPL currently expects to implement final rates by the first quarter of 2018. The IUB must issue a decision on requests for retail rate changes within 10 months of the date of the application for which changes are filed. For both the three and six months ended June 30, 2017, Alliant Energy and IPL recorded increases in electric margins of $20 million in conjunction with the interim retail electric base rate increase. WPL’s Retail Electric and Gas Rate Review (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement an increase in annual retail electric rates of $9 million, or approximately 1%, and an increase in annual retail gas base rates of $9 million, or approximately 13%. The $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. These increases were effective January 1, 2017 and extend through the end of 2018. For the three and six months ended June 30, 2017, Alliant Energy and WPL recorded increases in electric margins of $16 million and $38 million, and increases in gas margins of $2 million and $4 million, respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In July 2017, WPL filed a request with the PSCW to increase annual rates for WPL’s retail electric customers by $6 million, or approximately 1%, in 2018. The increase primarily reflects a change in expected fuel-related costs in 2018. Any rate changes granted from this request are expected to be effective January 1, 2018. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 are currently expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth and result in a deferral of under-collected fuel-related costs of $12 million for 2017. WPL’s application proposes to offset any deferral of projected under-collection of fuel-related costs from 2017 against the balance owed to customers for over-collected fuel-related costs for 2016 discussed below, and any remaining net balance at the end of 2017 would then be returned to, or collected from, customers in a future rate proceeding. Under WPL’s proposal, customer rates would not change during 2017 for the mid-year fuel-related cost adjustment. As of June 30, 2017, fuel-related costs for 2017 outside of the approved bandwidth were $8 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. As of June 30, 2017, fuel-related costs for 2016 outside of the approved bandwidth were $9 million and are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above.
IPL [Member] |
Public Utilities, General Disclosures [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets and Regulatory Liabilities - Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016
Tax-related |  | $1,100.7 |  |  |  | $1,055.6 |  |  |  | $1,063.3 |  |  |  | $1,022.4 |  |  |  | $37.4 |  |  |  | $33.2 |
Pension and OPEB costs | 558.3 |  |  | 578.7 |  |  | 284.2 |  |  | 294.0 |  |  | 274.1 |  |  | 284.7 |
Asset retirement obligations | 108.7 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.2 |  |  | 41.6 |
EGUs retired early | 78.2 |  |  | 41.4 |  |  | 40.3 |  |  | — |  |  | 37.9 |  |  | 41.4 |
Derivatives | 47.9 |  |  | 30.7 |  |  | 18.1 |  |  | 10.0 |  |  | 29.8 |  |  | 20.7 |
Emission allowances | 25.9 |  |  | 26.2 |  |  | 25.9 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.3 |  |  | 76.6 |  |  | 52.3 |  |  | 41.9 |  |  | 44.0 |  |  | 34.7 |
|  | $2,016.0 |  |  |  | $1,915.1 |  |  |  | $1,556.6 |  |  |  | $1,458.8 |  |  |  | $459.4 |  |  |  | $456.3 |
Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016
Cost of removal obligations |  | $415.3 |  |  |  | $411.6 |  |  |  | $273.0 |  |  |  | $269.4 |  |  |  | $142.3 |  |  |  | $142.2 |
Electric transmission cost recovery | 118.5 |  |  | 72.0 |  |  | 61.7 |  |  | 35.7 |  |  | 56.8 |  |  | 36.3 |
IPL’s tax benefit riders | 47.2 |  |  | 83.5 |  |  | 47.2 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 27.5 |  |  | 30.8 |  |  | 15.9 |  |  | 17.8 |  |  | 11.6 |  |  | 13.0 |
Energy efficiency cost recovery | 19.6 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.6 |  |  | 20.5 |
Derivatives | 11.4 |  |  | 31.5 |  |  | 7.1 |  |  | 12.1 |  |  | 4.3 |  |  | 19.4 |
Other | 26.3 |  |  | 31.1 |  |  | 10.8 |  |  | 12.3 |  |  | 15.5 |  |  | 18.8 |
|  | $665.8 |  |  |  | $681.0 |  |  |  | $415.7 |  |  |  | $430.8 |  |  |  | $250.1 |  |  |  | $250.2 |
Tax-related - Alliant Energy’s and IPL’s tax-related regulatory assets are generally impacted by certain property-related differences at IPL for which deferred tax is not recorded in the income statement pursuant to Iowa rate-making principles. Deferred tax amounts for such property-related differences at IPL are recorded to regulatory assets, along with the necessary revenue requirement tax gross-ups. During the six months ended June 30, 2017, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL is currently earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL has requested continued recovery of the remaining net book value of these EGUs from both its retail and wholesale customers over a 10-year period from the IUB and FERC, with decisions currently expected in 2018 and 2017, respectively. Derivatives - Refer to Note 11 for discussion of derivative assets and derivative liabilities. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC LLC to determine electric transmission costs billed to utilities, including IPL and WPL. In September 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32%, excluding any incentive adders granted by FERC, effective September 28, 2016, and for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In the first half of 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million, $39 million and $11 million, respectively, after final true-ups. IPL and WPL each recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC is currently being refunded to its retail customers in 2017, beginning May 2017. WPL’s retail portion of the refund from ATC LLC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in the first quarter of 2017. IPL’s tax benefit riders - IPL’s tax benefit riders utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures and cost of removal expenditures. For the six months ended June 30, 2017, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $36 million as follows (in millions):
Electric tax benefit rider credits |  | $33 |
Gas tax benefit rider credits | 3 |
|  | $36 |
Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers by $176 million, or approximately 12%. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million, or approximately 7%, on an annual basis, was implemented effective April 13, 2017, without regulatory review, and will be subject to refund pending determination of final rates. Tax benefit rider credits and MISO transmission owner return on equity refunds are expected to reduce the effect of the rate increase on customer bills in 2017 and 2018. IPL currently expects to implement final rates by the first quarter of 2018. The IUB must issue a decision on requests for retail rate changes within 10 months of the date of the application for which changes are filed. For both the three and six months ended June 30, 2017, Alliant Energy and IPL recorded increases in electric margins of $20 million in conjunction with the interim retail electric base rate increase. WPL’s Retail Electric and Gas Rate Review (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement an increase in annual retail electric rates of $9 million, or approximately 1%, and an increase in annual retail gas base rates of $9 million, or approximately 13%. The $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. These increases were effective January 1, 2017 and extend through the end of 2018. For the three and six months ended June 30, 2017, Alliant Energy and WPL recorded increases in electric margins of $16 million and $38 million, and increases in gas margins of $2 million and $4 million, respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In July 2017, WPL filed a request with the PSCW to increase annual rates for WPL’s retail electric customers by $6 million, or approximately 1%, in 2018. The increase primarily reflects a change in expected fuel-related costs in 2018. Any rate changes granted from this request are expected to be effective January 1, 2018. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 are currently expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth and result in a deferral of under-collected fuel-related costs of $12 million for 2017. WPL’s application proposes to offset any deferral of projected under-collection of fuel-related costs from 2017 against the balance owed to customers for over-collected fuel-related costs for 2016 discussed below, and any remaining net balance at the end of 2017 would then be returned to, or collected from, customers in a future rate proceeding. Under WPL’s proposal, customer rates would not change during 2017 for the mid-year fuel-related cost adjustment. As of June 30, 2017, fuel-related costs for 2017 outside of the approved bandwidth were $8 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. As of June 30, 2017, fuel-related costs for 2016 outside of the approved bandwidth were $9 million and are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above.
WPL [Member] |
Public Utilities, General Disclosures [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets and Regulatory Liabilities - Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016
Tax-related |  | $1,100.7 |  |  |  | $1,055.6 |  |  |  | $1,063.3 |  |  |  | $1,022.4 |  |  |  | $37.4 |  |  |  | $33.2 |
Pension and OPEB costs | 558.3 |  |  | 578.7 |  |  | 284.2 |  |  | 294.0 |  |  | 274.1 |  |  | 284.7 |
Asset retirement obligations | 108.7 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.2 |  |  | 41.6 |
EGUs retired early | 78.2 |  |  | 41.4 |  |  | 40.3 |  |  | — |  |  | 37.9 |  |  | 41.4 |
Derivatives | 47.9 |  |  | 30.7 |  |  | 18.1 |  |  | 10.0 |  |  | 29.8 |  |  | 20.7 |
Emission allowances | 25.9 |  |  | 26.2 |  |  | 25.9 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.3 |  |  | 76.6 |  |  | 52.3 |  |  | 41.9 |  |  | 44.0 |  |  | 34.7 |
|  | $2,016.0 |  |  |  | $1,915.1 |  |  |  | $1,556.6 |  |  |  | $1,458.8 |  |  |  | $459.4 |  |  |  | $456.3 |
Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016 |  | June 30, 2017 |  | December 31, 2016
Cost of removal obligations |  | $415.3 |  |  |  | $411.6 |  |  |  | $273.0 |  |  |  | $269.4 |  |  |  | $142.3 |  |  |  | $142.2 |
Electric transmission cost recovery | 118.5 |  |  | 72.0 |  |  | 61.7 |  |  | 35.7 |  |  | 56.8 |  |  | 36.3 |
IPL’s tax benefit riders | 47.2 |  |  | 83.5 |  |  | 47.2 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 27.5 |  |  | 30.8 |  |  | 15.9 |  |  | 17.8 |  |  | 11.6 |  |  | 13.0 |
Energy efficiency cost recovery | 19.6 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.6 |  |  | 20.5 |
Derivatives | 11.4 |  |  | 31.5 |  |  | 7.1 |  |  | 12.1 |  |  | 4.3 |  |  | 19.4 |
Other | 26.3 |  |  | 31.1 |  |  | 10.8 |  |  | 12.3 |  |  | 15.5 |  |  | 18.8 |
|  | $665.8 |  |  |  | $681.0 |  |  |  | $415.7 |  |  |  | $430.8 |  |  |  | $250.1 |  |  |  | $250.2 |
Tax-related - Alliant Energy’s and IPL’s tax-related regulatory assets are generally impacted by certain property-related differences at IPL for which deferred tax is not recorded in the income statement pursuant to Iowa rate-making principles. Deferred tax amounts for such property-related differences at IPL are recorded to regulatory assets, along with the necessary revenue requirement tax gross-ups. During the six months ended June 30, 2017, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL is currently earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL has requested continued recovery of the remaining net book value of these EGUs from both its retail and wholesale customers over a 10-year period from the IUB and FERC, with decisions currently expected in 2018 and 2017, respectively. Derivatives - Refer to Note 11 for discussion of derivative assets and derivative liabilities. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC LLC to determine electric transmission costs billed to utilities, including IPL and WPL. In September 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32%, excluding any incentive adders granted by FERC, effective September 28, 2016, and for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In the first half of 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million, $39 million and $11 million, respectively, after final true-ups. IPL and WPL each recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC is currently being refunded to its retail customers in 2017, beginning May 2017. WPL’s retail portion of the refund from ATC LLC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in the first quarter of 2017. IPL’s tax benefit riders - IPL’s tax benefit riders utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures and cost of removal expenditures. For the six months ended June 30, 2017, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $36 million as follows (in millions):
Electric tax benefit rider credits |  | $33 |
Gas tax benefit rider credits | 3 |
|  | $36 |
Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers by $176 million, or approximately 12%. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million, or approximately 7%, on an annual basis, was implemented effective April 13, 2017, without regulatory review, and will be subject to refund pending determination of final rates. Tax benefit rider credits and MISO transmission owner return on equity refunds are expected to reduce the effect of the rate increase on customer bills in 2017 and 2018. IPL currently expects to implement final rates by the first quarter of 2018. The IUB must issue a decision on requests for retail rate changes within 10 months of the date of the application for which changes are filed. For both the three and six months ended June 30, 2017, Alliant Energy and IPL recorded increases in electric margins of $20 million in conjunction with the interim retail electric base rate increase. WPL’s Retail Electric and Gas Rate Review (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement an increase in annual retail electric rates of $9 million, or approximately 1%, and an increase in annual retail gas base rates of $9 million, or approximately 13%. The $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. These increases were effective January 1, 2017 and extend through the end of 2018. For the three and six months ended June 30, 2017, Alliant Energy and WPL recorded increases in electric margins of $16 million and $38 million, and increases in gas margins of $2 million and $4 million, respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In July 2017, WPL filed a request with the PSCW to increase annual rates for WPL’s retail electric customers by $6 million, or approximately 1%, in 2018. The increase primarily reflects a change in expected fuel-related costs in 2018. Any rate changes granted from this request are expected to be effective January 1, 2018. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 are currently expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth and result in a deferral of under-collected fuel-related costs of $12 million for 2017. WPL’s application proposes to offset any deferral of projected under-collection of fuel-related costs from 2017 against the balance owed to customers for over-collected fuel-related costs for 2016 discussed below, and any remaining net balance at the end of 2017 would then be returned to, or collected from, customers in a future rate proceeding. Under WPL’s proposal, customer rates would not change during 2017 for the mid-year fuel-related cost adjustment. As of June 30, 2017, fuel-related costs for 2017 outside of the approved bandwidth were $8 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. As of June 30, 2017, fuel-related costs for 2016 outside of the approved bandwidth were $9 million and are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above.

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