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Regulatory Matters (Tables) | 9 Months Ended
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Sep. 30, 2021
Regulatory Assets and Liabilities Disclosure [Abstract] |
Schedule of Regulatory Assets and Liabilities | The following presents our current and long-term regulatory assets and liabilities as of September 30, 2021 and December 31, 2020 and the period we expect to recover or refund such amounts:
| Period of |  | September 30, 2021 |  | December 31, 2020
(in thousands) | Recovery/Refund |  | Current |  | Long-Term |  | Current |  | Long Term
Regulatory Assets |  |  |  |  |  |  |  |  |
Pension and Other Postretirement Benefit Plans 1 | Various |  | $ | 11,037 |  |  | $ | 139,467 |  |  | $ | 11,037 |  |  | $ | 146,071 |
Alternative Revenue Program Riders 2 | Up to 3 years |  | 7,229 |  |  | 8,225 |  |  | 8,871 |  |  | 9,373 |
Asset Retirement Obligations 1 | Asset lives |  | — |  |  | 7,065 |  |  | — |  |  | 8,462 |
ISO Cost Recovery Trackers 1 | Up to 2 years |  | 270 |  |  | 1,027 |  |  | 1,079 |  |  | 867 |
Unrecovered Project Costs 1 | Up to 3 years |  | 3,227 |  |  | 1,456 |  |  | 361 |  |  | 2,989 |
Deferred Rate Case Expenses 1 | Various |  | 637 |  |  | 936 |  |  | 360 |  |  | 230 |
Debt Reacquisition Premiums 1 | Up to 12 years |  | 117 |  |  | 262 |  |  | 192 |  |  | 341 |
Other 1 | Various |  | — |  |  | 77 |  |  | — |  |  | 62 |
Total Regulatory Assets |  |  | $ | 22,517 |  |  | $ | 158,515 |  |  | $ | 21,900 |  |  | $ | 168,395 |
Regulatory Liabilities |  |  |  |  |  |  |  |  |
Deferred Income Taxes | Asset lives |  | $ | — |  |  | $ | 130,555 |  |  | $ | — |  |  | $ | 134,719 |
Plant Removal Obligations | Asset lives |  | 5,893 |  |  | 96,734 |  |  | — |  |  | 98,707 |
Fuel Clause Adjustments | Up to 1 year |  | 5,692 |  |  | — |  |  | 10,947 |  |  | — |
Alternative Revenue Program Riders | Various |  | 5,265 |  |  | 2,507 |  |  | 3,581 |  |  | 470 |
Pension and Other Postretirement Benefit Plans | Up to 1 year |  | 1,959 |  |  | — |  |  | 1,959 |  |  | — |
Derivative Instruments | Various |  | 6,136 |  |  | 787 |  |  | — |  |  | — |
Other | Various |  | 378 |  |  | 150 |  |  | 176 |  |  | 77 |
Total Regulatory Liabilities |  |  | $ | 25,323 |  |  | $ | 230,733 |  |  | $ | 16,663 |  |  | $ | 233,973 |
1 Costs subject to recovery without a rate of return. |  |  |  |  |  |  |  |  |
2 Amount eligible for recovery includes an incentive or rate of return. |  |  |  |  |  |  |  |  |

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