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Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Jun. 30, 2023 | Mar. 31, 2023 | Sep. 30, 2022 | Jun. 30, 2022 | Mar. 31, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Accumulated other comprehensive loss [Roll Forward] |  |  |  |  |  |  |  |
Balance | $ 2,706,193 | $ 3,575,382 | $ 3,587,129 | $ 3,386,810 | $ 3,360,718 | $ 3,382,874 | $ 3,587,129 | $ 3,382,874
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax | 0 |  |  | 0 |  |  | 0 | (3,265)
Other comprehensive income (loss) | (62) | 363 | 217 | 285 | (2,796) | 269 | 518 | (2,242)
Balance | 2,757,434 | 2,706,193 | 3,575,382 | 3,491,858 | 3,386,810 | 3,360,718 | 2,757,434 | 3,491,858
Net unrealized gain (loss) on derivative instruments qualifying as hedges |  |  |  |  |  |  |  |
Accumulated other comprehensive loss [Roll Forward] |  |  |  |  |  |  |  |
Balance | 0 | (91) | (125) | (315) | (426) | (538) | (125) | (538)
Other comprehensive loss before reclassifications | 0 | 0 | 0 | 0 | 0 | 0 |  |
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax |  |  |  |  | 0 |  |  |
Amounts reclassified from accumulated other comprehensive loss | 0 | 47 | 34 | 111 | 111 | 112 |  |
Other comprehensive income (loss) | 0 | 47 | 34 | 111 | 111 | 112 |  |
Amounts reclassified related to the separation of Knife River |  | 44 |  |  |  |  |  |
Balance | 0 | 0 | (91) | (204) | (315) | (426) | 0 | (204)
Postretirement liability adjustment |  |  |  |  |  |  |  |
Accumulated other comprehensive loss [Roll Forward] |  |  |  |  |  |  |  |
Balance | (17,151) | (29,800) | (29,900) | (42,820) | (40,016) | (40,461) | (29,900) | (40,461)
Other comprehensive loss before reclassifications | 0 | 0 | 0 | 0 | 0 | 0 |  |
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax |  |  |  |  | (3,265) |  |  |
Amounts reclassified from accumulated other comprehensive loss | (6) | 387 | 100 | 461 | 461 | 445 |  |
Other comprehensive income (loss) | (6) | 387 | 100 | 461 | (2,804) | 445 |  |
Amounts reclassified related to the separation of Knife River |  | 12,262 |  |  |  |  |  |
Balance | (17,157) | (17,151) | (29,800) | (42,359) | (42,820) | (40,016) | (17,157) | (42,359)
Net unrealized gain (loss) on available-for-sale investments |  |  |  |  |  |  |  |
Accumulated other comprehensive loss [Roll Forward] |  |  |  |  |  |  |  |
Balance | (546) | (475) | (558) | (396) | (293) | (5) | (558) | (5)
Other comprehensive loss before reclassifications | (66) | (84) | 70 | 329 | (128) | (320) |  |
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax | 269 |  |  |  | 0 |  |  |
Amounts reclassified from accumulated other comprehensive loss | 10 | 13 | 13 | 42 | 25 | 32 |  |
Other comprehensive income (loss) | (56) | (71) | 83 | (287) | (103) | (288) |  |
Amounts reclassified related to the separation of Knife River |  | 0 |  |  |  |  |  |
Balance | (602) | (546) | (475) | (683) | (396) | (293) | (602) | (683)
Total accumulated other comprehensive loss |  |  |  |  |  |  |  |
Accumulated other comprehensive loss [Roll Forward] |  |  |  |  |  |  |  |
Balance | (17,697) | (30,366) | (30,583) | (43,531) | (40,735) | (41,004) | (30,583) | (41,004)
Other comprehensive loss before reclassifications | (66) | (84) | 70 | 329 | (128) | (320) |  |
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax |  |  |  |  | (3,265) |  |  |
Amounts reclassified from accumulated other comprehensive loss | 4 | 447 | 147 | 614 | 597 | 589 |  |
Other comprehensive income (loss) | (62) | 363 | 217 | 285 | (2,796) | 269 |  |
Amounts reclassified related to the separation of Knife River |  | 12,306 |  |  |  |  |  |
Balance | $ (17,759) | $ (17,697) | $ (30,366) | $ (43,246) | $ (43,531) | $ (40,735) | $ (17,759) | $ (43,246)

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