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Comprehensive income (loss) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance |  |  | $ 2,429,043 | $ 2,316,244
Other comprehensive income | $ 516 | $ 448 | 1,674 | 392
Balance | 2,521,293 | 2,353,917 | 2,521,293 | 2,353,917
Net unrealized gain (loss) on derivative instruments qualifying as hedges |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (2,136) | (2,117) | (1,934) | (2,300)
Other comprehensive loss before reclassifications | 0 | 0 | 0 | 0
Amounts reclassified from accumulated other comprehensive loss | 92 | 92 | 279 | 275
Amounts reclassified to accumulated other comprehensive loss from regulatory asset |  |  |  | 0
Other comprehensive income | 92 | 92 | 279 | 275
Balance | (2,044) | (2,025) | (2,044) | (2,025)
Postretirement liability adjustment |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (41,816) | (33,469) | (35,163) | (33,221)
Other comprehensive loss before reclassifications | 0 | 0 | 0 | 0
Amounts reclassified from accumulated other comprehensive loss | 442 | 333 | 1,309 | 1,002
Amounts reclassified to accumulated other comprehensive loss from regulatory asset |  |  |  | (917)
Other comprehensive income | 442 | 333 | 1,309 | 85
Balance | (41,374) | (33,136) | (41,374) | (33,136)
Foreign currency translation adjustment |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | 0 | (155) | (155) | (149)
Other comprehensive loss before reclassifications | 0 | 15 | (61) | 9
Amounts reclassified from accumulated other comprehensive loss | 0 | 0 | 249 | 0
Amounts reclassified to accumulated other comprehensive loss from regulatory asset |  |  |  | 0
Other comprehensive income | 0 | 15 | 188 | 9
Balance | 0 | (140) | 0 | (140)
Net unrealized gain (loss) on available-for-sale investments |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (183) | (48) | (82) | (63)
Other comprehensive loss before reclassifications | (51) | (19) | (199) | (70)
Amounts reclassified from accumulated other comprehensive loss | 33 | 27 | 97 | 93
Amounts reclassified to accumulated other comprehensive loss from regulatory asset |  |  |  | 0
Other comprehensive income | (18) | 8 | (102) | 23
Balance | (201) | (40) | (201) | (40)
Total accumulated other comprehensive loss |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (44,135) | (35,789) | (37,334) | (35,733)
Other comprehensive loss before reclassifications | (51) | (4) | (260) | (61)
Amounts reclassified from accumulated other comprehensive loss | 567 | 452 | 1,934 | 1,370
Amounts reclassified to accumulated other comprehensive loss from regulatory asset |  |  |  | (917)
Other comprehensive income | 516 | 448 | 1,674 | 392
Balance | $ (43,619) | $ (35,341) | (43,619) | $ (35,341)
Adjustments for new accounting principle, early adoption | Tax Cuts and Jobs Act | Net unrealized gain (loss) on derivative instruments qualifying as hedges |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss |  |  | (389) |
Adjustments for new accounting principle, early adoption | Tax Cuts and Jobs Act | Postretirement liability adjustment |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss |  |  | (7,520) |
Adjustments for new accounting principle, early adoption | Tax Cuts and Jobs Act | Foreign currency translation adjustment |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss |  |  | (33) |
Adjustments for new accounting principle, early adoption | Tax Cuts and Jobs Act | Net unrealized gain (loss) on available-for-sale investments |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss |  |  | (17) |
Adjustments for new accounting principle, early adoption | Tax Cuts and Jobs Act | Total accumulated other comprehensive loss |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss |  |  | $ (7,959) |

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