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Other Comprehensive Income (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Interest expense | $ 154,112 | $ 144,931 | $ 139,291
Fuel, purchased power and cost of natural gas sold | 741,934 | 492,404 | 570,829
Operations and maintenance | 501,690 | 495,404 | 495,994
Income Tax Expense (Benefit) | (7,169) | (32,918) | (29,580)
Net income | 251,260 | 242,763 | $ 213,322
Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Net income | (2,280) | (4,418) |
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Income before income taxes | (800) | (3,452) |
Income Tax Expense (Benefit) | 175 | 383 |
Net income | (625) | (3,069) |
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Interest Rate Swap |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Interest expense | (2,851) | (2,851) |
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Commodity Contract |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Fuel, purchased power and cost of natural gas sold | 2,051 | (601) |
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost | Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Operations and maintenance | 98 | 103 |
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Operations and maintenance | (2,391) | (2,387) |
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] |  |  |
Income before income taxes | (2,293) | (2,284) |
Income Tax Expense (Benefit) | 638 | 935 |
Net income | $ (1,655) | $ (1,349) |

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