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Regulatory Matters (Tables) | 3 Months Ended
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Mar. 31, 2016
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015
Tax-related |  | $1,002.7 |  |  |  | $987.7 |  |  |  | $972.0 |  |  |  | $958.2 |  |  |  | $30.7 |  |  |  | $29.5 |
Pension and OPEB costs | 570.0 |  |  | 579.5 |  |  | 293.6 |  |  | 298.1 |  |  | 276.4 |  |  | 281.4 |
AROs | 96.1 |  |  | 92.4 |  |  | 54.1 |  |  | 50.8 |  |  | 42.0 |  |  | 41.6 |
Derivatives | 86.6 |  |  | 70.6 |  |  | 25.5 |  |  | 28.2 |  |  | 61.1 |  |  | 42.4 |
WPL’s EGUs retired early | 43.2 |  |  | 45.0 |  |  | — |  |  | — |  |  | 43.2 |  |  | 45.0 |
Emission allowances | 26.7 |  |  | 26.9 |  |  | 26.7 |  |  | 26.9 |  |  | — |  |  | — |
Commodity cost recovery | 26.3 |  |  | 35.9 |  |  | 0.3 |  |  | 2.8 |  |  | 26.0 |  |  | 33.1 |
Other | 65.6 |  |  | 70.6 |  |  | 33.7 |  |  | 37.6 |  |  | 31.9 |  |  | 33.0 |
|  | $1,917.2 |  |  |  | $1,908.6 |  |  |  | $1,405.9 |  |  |  | $1,402.6 |  |  |  | $511.3 |  |  |  | $506.0 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015
Cost of removal obligations |  | $408.1 |  |  |  | $406.0 |  |  |  | $263.1 |  |  |  | $260.4 |  |  |  | $145.0 |  |  |  | $145.6 |
IPL’s tax benefit riders | 141.1 |  |  | 159.2 |  |  | 141.1 |  |  | 159.2 |  |  | — |  |  | — |
Electric transmission cost recovery | 51.4 |  |  | 43.5 |  |  | 25.5 |  |  | 21.9 |  |  | 25.9 |  |  | 21.6 |
Energy efficiency cost recovery | 41.6 |  |  | 48.3 |  |  | — |  |  | — |  |  | 41.6 |  |  | 48.3 |
Commodity cost recovery | 35.2 |  |  | 37.6 |  |  | 20.4 |  |  | 23.5 |  |  | 14.8 |  |  | 14.1 |
Other | 33.6 |  |  | 43.1 |  |  | 18.1 |  |  | 24.2 |  |  | 15.5 |  |  | 18.9 |
|  | $711.0 |  |  |  | $737.7 |  |  |  | $468.2 |  |  |  | $489.2 |  |  |  | $242.8 |  |  |  | $248.5 |
Tax Benefit Riders | For the three months ended March 31, 2016, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $18 million as follows (in millions):
Electric tax benefit rider credits |  | $15 |
Gas tax benefit rider credits | 3 |
|  | $18 |
Customer Billing Credits | For the three months ended March 31, IPL recorded billing credits to reduce retail electric customers’ bills as follows (in millions):
| 2016 |  | 2015
Billing credits to reduce retail electric customers’ bills | $2 |  |  | $6 |
IPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015
Tax-related |  | $1,002.7 |  |  |  | $987.7 |  |  |  | $972.0 |  |  |  | $958.2 |  |  |  | $30.7 |  |  |  | $29.5 |
Pension and OPEB costs | 570.0 |  |  | 579.5 |  |  | 293.6 |  |  | 298.1 |  |  | 276.4 |  |  | 281.4 |
AROs | 96.1 |  |  | 92.4 |  |  | 54.1 |  |  | 50.8 |  |  | 42.0 |  |  | 41.6 |
Derivatives | 86.6 |  |  | 70.6 |  |  | 25.5 |  |  | 28.2 |  |  | 61.1 |  |  | 42.4 |
WPL’s EGUs retired early | 43.2 |  |  | 45.0 |  |  | — |  |  | — |  |  | 43.2 |  |  | 45.0 |
Emission allowances | 26.7 |  |  | 26.9 |  |  | 26.7 |  |  | 26.9 |  |  | — |  |  | — |
Commodity cost recovery | 26.3 |  |  | 35.9 |  |  | 0.3 |  |  | 2.8 |  |  | 26.0 |  |  | 33.1 |
Other | 65.6 |  |  | 70.6 |  |  | 33.7 |  |  | 37.6 |  |  | 31.9 |  |  | 33.0 |
|  | $1,917.2 |  |  |  | $1,908.6 |  |  |  | $1,405.9 |  |  |  | $1,402.6 |  |  |  | $511.3 |  |  |  | $506.0 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015
Cost of removal obligations |  | $408.1 |  |  |  | $406.0 |  |  |  | $263.1 |  |  |  | $260.4 |  |  |  | $145.0 |  |  |  | $145.6 |
IPL’s tax benefit riders | 141.1 |  |  | 159.2 |  |  | 141.1 |  |  | 159.2 |  |  | — |  |  | — |
Electric transmission cost recovery | 51.4 |  |  | 43.5 |  |  | 25.5 |  |  | 21.9 |  |  | 25.9 |  |  | 21.6 |
Energy efficiency cost recovery | 41.6 |  |  | 48.3 |  |  | — |  |  | — |  |  | 41.6 |  |  | 48.3 |
Commodity cost recovery | 35.2 |  |  | 37.6 |  |  | 20.4 |  |  | 23.5 |  |  | 14.8 |  |  | 14.1 |
Other | 33.6 |  |  | 43.1 |  |  | 18.1 |  |  | 24.2 |  |  | 15.5 |  |  | 18.9 |
|  | $711.0 |  |  |  | $737.7 |  |  |  | $468.2 |  |  |  | $489.2 |  |  |  | $242.8 |  |  |  | $248.5 |
Tax Benefit Riders | For the three months ended March 31, 2016, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $18 million as follows (in millions):
Electric tax benefit rider credits |  | $15 |
Gas tax benefit rider credits | 3 |
|  | $18 |
Customer Billing Credits | For the three months ended March 31, IPL recorded billing credits to reduce retail electric customers’ bills as follows (in millions):
| 2016 |  | 2015
Billing credits to reduce retail electric customers’ bills | $2 |  |  | $6 |
WPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015
Tax-related |  | $1,002.7 |  |  |  | $987.7 |  |  |  | $972.0 |  |  |  | $958.2 |  |  |  | $30.7 |  |  |  | $29.5 |
Pension and OPEB costs | 570.0 |  |  | 579.5 |  |  | 293.6 |  |  | 298.1 |  |  | 276.4 |  |  | 281.4 |
AROs | 96.1 |  |  | 92.4 |  |  | 54.1 |  |  | 50.8 |  |  | 42.0 |  |  | 41.6 |
Derivatives | 86.6 |  |  | 70.6 |  |  | 25.5 |  |  | 28.2 |  |  | 61.1 |  |  | 42.4 |
WPL’s EGUs retired early | 43.2 |  |  | 45.0 |  |  | — |  |  | — |  |  | 43.2 |  |  | 45.0 |
Emission allowances | 26.7 |  |  | 26.9 |  |  | 26.7 |  |  | 26.9 |  |  | — |  |  | — |
Commodity cost recovery | 26.3 |  |  | 35.9 |  |  | 0.3 |  |  | 2.8 |  |  | 26.0 |  |  | 33.1 |
Other | 65.6 |  |  | 70.6 |  |  | 33.7 |  |  | 37.6 |  |  | 31.9 |  |  | 33.0 |
|  | $1,917.2 |  |  |  | $1,908.6 |  |  |  | $1,405.9 |  |  |  | $1,402.6 |  |  |  | $511.3 |  |  |  | $506.0 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015 |  | March 31, 2016 |  | December 31, 2015
Cost of removal obligations |  | $408.1 |  |  |  | $406.0 |  |  |  | $263.1 |  |  |  | $260.4 |  |  |  | $145.0 |  |  |  | $145.6 |
IPL’s tax benefit riders | 141.1 |  |  | 159.2 |  |  | 141.1 |  |  | 159.2 |  |  | — |  |  | — |
Electric transmission cost recovery | 51.4 |  |  | 43.5 |  |  | 25.5 |  |  | 21.9 |  |  | 25.9 |  |  | 21.6 |
Energy efficiency cost recovery | 41.6 |  |  | 48.3 |  |  | — |  |  | — |  |  | 41.6 |  |  | 48.3 |
Commodity cost recovery | 35.2 |  |  | 37.6 |  |  | 20.4 |  |  | 23.5 |  |  | 14.8 |  |  | 14.1 |
Other | 33.6 |  |  | 43.1 |  |  | 18.1 |  |  | 24.2 |  |  | 15.5 |  |  | 18.9 |
|  | $711.0 |  |  |  | $737.7 |  |  |  | $468.2 |  |  |  | $489.2 |  |  |  | $242.8 |  |  |  | $248.5 |

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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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