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Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | $ 3,079,105 | $ 2,847,246
Other comprehensive income (loss) | 568 | 707
Balance | 3,100,306 | 2,836,592
Net unrealized gain (loss) on derivative instruments qualifying as hedges |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | (984) | (1,430)
Other comprehensive income (loss) before reclassifications | 0 | 0
Amounts reclassified (to) from accumulated other comprehensive loss | 111 | 111
Other comprehensive income (loss) | 111 | 111
Balance | (873) | (1,319)
Postretirement liability adjustment |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | (47,207) | (40,734)
Other comprehensive income (loss) before reclassifications | 0 | 0
Amounts reclassified (to) from accumulated other comprehensive loss | 466 | 462
Other comprehensive income (loss) | 466 | 462
Balance | (46,741) | (40,272)
Net unrealized gain (loss) on available-for-sale investments |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | 113 | 62
Other comprehensive income (loss) before reclassifications | (44) | 135
Amounts reclassified (to) from accumulated other comprehensive loss | 35 | (1)
Other comprehensive income (loss) | (9) | 134
Balance | 104 | 196
Total accumulated other comprehensive loss |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | (48,078) | (42,102)
Other comprehensive income (loss) before reclassifications | (44) | 135
Amounts reclassified (to) from accumulated other comprehensive loss | 612 | 572
Other comprehensive income (loss) | 568 | 707
Balance | $ (47,510) | $ (41,395)

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