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Regulatory Matters (Tables) | 9 Months Ended
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Sep. 30, 2015
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014
Tax-related |  | $976.5 |  |  |  | $955.3 |  |  |  | $947.3 |  |  |  | $928.0 |  |  |  | $29.2 |  |  |  | $27.3 |
Pension and OPEB costs | 548.4 |  |  | 570.2 |  |  | 278.6 |  |  | 287.9 |  |  | 269.8 |  |  | 282.3 |
AROs | 81.8 |  |  | 73.7 |  |  | 47.6 |  |  | 41.4 |  |  | 34.2 |  |  | 32.3 |
Derivatives | 68.4 |  |  | 46.9 |  |  | 29.3 |  |  | 28.0 |  |  | 39.1 |  |  | 18.9 |
Commodity cost recovery | 33.2 |  |  | 31.1 |  |  | 1.2 |  |  | 0.4 |  |  | 32.0 |  |  | 30.7 |
Emission allowances | 26.5 |  |  | 27.4 |  |  | 26.5 |  |  | 27.4 |  |  | — |  |  | — |
Other | 73.1 |  |  | 79.1 |  |  | 39.7 |  |  | 44.8 |  |  | 33.4 |  |  | 34.3 |
|  | $1,807.9 |  |  |  | $1,783.7 |  |  |  | $1,370.2 |  |  |  | $1,357.9 |  |  |  | $437.7 |  |  |  | $425.8 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014
Cost of removal obligations |  | $406.2 |  |  |  | $421.7 |  |  |  | $261.3 |  |  |  | $279.1 |  |  |  | $144.9 |  |  |  | $142.6 |
IPL’s tax benefit riders | 179.5 |  |  | 243.0 |  |  | 179.5 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 53.0 |  |  | 64.3 |  |  | — |  |  | — |  |  | 53.0 |  |  | 64.3 |
Commodity cost recovery | 33.8 |  |  | 15.4 |  |  | 22.9 |  |  | 15.1 |  |  | 10.9 |  |  | 0.3 |
Electric transmission cost recovery | 31.4 |  |  | 19.4 |  |  | 15.8 |  |  | 19.4 |  |  | 15.6 |  |  | — |
Other | 40.2 |  |  | 57.4 |  |  | 23.0 |  |  | 26.9 |  |  | 17.2 |  |  | 30.5 |
|  | $744.1 |  |  |  | $821.2 |  |  |  | $502.5 |  |  |  | $583.5 |  |  |  | $241.6 |  |  |  | $237.7 |
Tax Benefit Riders | For the nine months ended September 30, 2015, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $64 million as follows (in millions):
Electric tax benefit rider credits |  | $55 |
Gas tax benefit rider credits | 9 |
|  | $64 |
Customer Billing Credits | For the three and nine months ended September 30, IPL recorded billing credits to reduce retail electric customers’ bills as follows (in millions):
| Three Months |  | Nine Months
| 2015 |  | 2014 |  | 2015 |  | 2014
Billing credits to reduce retail electric customers’ bills | $7 |  |  | $26 |  |  |  | $19 |  |  |  | $46 |
IPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014
Tax-related |  | $976.5 |  |  |  | $955.3 |  |  |  | $947.3 |  |  |  | $928.0 |  |  |  | $29.2 |  |  |  | $27.3 |
Pension and OPEB costs | 548.4 |  |  | 570.2 |  |  | 278.6 |  |  | 287.9 |  |  | 269.8 |  |  | 282.3 |
AROs | 81.8 |  |  | 73.7 |  |  | 47.6 |  |  | 41.4 |  |  | 34.2 |  |  | 32.3 |
Derivatives | 68.4 |  |  | 46.9 |  |  | 29.3 |  |  | 28.0 |  |  | 39.1 |  |  | 18.9 |
Commodity cost recovery | 33.2 |  |  | 31.1 |  |  | 1.2 |  |  | 0.4 |  |  | 32.0 |  |  | 30.7 |
Emission allowances | 26.5 |  |  | 27.4 |  |  | 26.5 |  |  | 27.4 |  |  | — |  |  | — |
Other | 73.1 |  |  | 79.1 |  |  | 39.7 |  |  | 44.8 |  |  | 33.4 |  |  | 34.3 |
|  | $1,807.9 |  |  |  | $1,783.7 |  |  |  | $1,370.2 |  |  |  | $1,357.9 |  |  |  | $437.7 |  |  |  | $425.8 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014
Cost of removal obligations |  | $406.2 |  |  |  | $421.7 |  |  |  | $261.3 |  |  |  | $279.1 |  |  |  | $144.9 |  |  |  | $142.6 |
IPL’s tax benefit riders | 179.5 |  |  | 243.0 |  |  | 179.5 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 53.0 |  |  | 64.3 |  |  | — |  |  | — |  |  | 53.0 |  |  | 64.3 |
Commodity cost recovery | 33.8 |  |  | 15.4 |  |  | 22.9 |  |  | 15.1 |  |  | 10.9 |  |  | 0.3 |
Electric transmission cost recovery | 31.4 |  |  | 19.4 |  |  | 15.8 |  |  | 19.4 |  |  | 15.6 |  |  | — |
Other | 40.2 |  |  | 57.4 |  |  | 23.0 |  |  | 26.9 |  |  | 17.2 |  |  | 30.5 |
|  | $744.1 |  |  |  | $821.2 |  |  |  | $502.5 |  |  |  | $583.5 |  |  |  | $241.6 |  |  |  | $237.7 |
Tax Benefit Riders | For the nine months ended September 30, 2015, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $64 million as follows (in millions):
Electric tax benefit rider credits |  | $55 |
Gas tax benefit rider credits | 9 |
|  | $64 |
Customer Billing Credits | For the three and nine months ended September 30, IPL recorded billing credits to reduce retail electric customers’ bills as follows (in millions):
| Three Months |  | Nine Months
| 2015 |  | 2014 |  | 2015 |  | 2014
Billing credits to reduce retail electric customers’ bills | $7 |  |  | $26 |  |  |  | $19 |  |  |  | $46 |
WPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014
Tax-related |  | $976.5 |  |  |  | $955.3 |  |  |  | $947.3 |  |  |  | $928.0 |  |  |  | $29.2 |  |  |  | $27.3 |
Pension and OPEB costs | 548.4 |  |  | 570.2 |  |  | 278.6 |  |  | 287.9 |  |  | 269.8 |  |  | 282.3 |
AROs | 81.8 |  |  | 73.7 |  |  | 47.6 |  |  | 41.4 |  |  | 34.2 |  |  | 32.3 |
Derivatives | 68.4 |  |  | 46.9 |  |  | 29.3 |  |  | 28.0 |  |  | 39.1 |  |  | 18.9 |
Commodity cost recovery | 33.2 |  |  | 31.1 |  |  | 1.2 |  |  | 0.4 |  |  | 32.0 |  |  | 30.7 |
Emission allowances | 26.5 |  |  | 27.4 |  |  | 26.5 |  |  | 27.4 |  |  | — |  |  | — |
Other | 73.1 |  |  | 79.1 |  |  | 39.7 |  |  | 44.8 |  |  | 33.4 |  |  | 34.3 |
|  | $1,807.9 |  |  |  | $1,783.7 |  |  |  | $1,370.2 |  |  |  | $1,357.9 |  |  |  | $437.7 |  |  |  | $425.8 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014 |  | September 30, 2015 |  | December 31, 2014
Cost of removal obligations |  | $406.2 |  |  |  | $421.7 |  |  |  | $261.3 |  |  |  | $279.1 |  |  |  | $144.9 |  |  |  | $142.6 |
IPL’s tax benefit riders | 179.5 |  |  | 243.0 |  |  | 179.5 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 53.0 |  |  | 64.3 |  |  | — |  |  | — |  |  | 53.0 |  |  | 64.3 |
Commodity cost recovery | 33.8 |  |  | 15.4 |  |  | 22.9 |  |  | 15.1 |  |  | 10.9 |  |  | 0.3 |
Electric transmission cost recovery | 31.4 |  |  | 19.4 |  |  | 15.8 |  |  | 19.4 |  |  | 15.6 |  |  | — |
Other | 40.2 |  |  | 57.4 |  |  | 23.0 |  |  | 26.9 |  |  | 17.2 |  |  | 30.5 |
|  | $744.1 |  |  |  | $821.2 |  |  |  | $502.5 |  |  |  | $583.5 |  |  |  | $241.6 |  |  |  | $237.7 |

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- Definition Tabular disclosure of assets that are created when regulatory agencies permit public utilities to defer certain costs included in rate-setting to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44264-110382 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 715 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501251&loc=d3e52485-110419 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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