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Comprehensive income (loss) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance |  |  |  | $ 2,520,548
Amounts reclassified from accumulated other comprehensive loss | $ (452) | $ (361) | $ (1,370) | 730
Net current-period other comprehensive income (loss) | 448 | 315 | 392 | (743)
Balance |  | 2,285,061 |  | 2,285,061
Net unrealized gain (loss) on derivative instruments qualifying as hedges |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (2,117) | (2,484) | (2,300) | (2,667)
Other comprehensive loss before reclassifications | 0 | 0 | 0 | 0
Amounts reclassified from accumulated other comprehensive loss | 92 | 92 | 275 | 275
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  |  | 0 |
Net current-period other comprehensive income (loss) | 92 | 92 | 275 | 275
Balance | (2,025) | (2,392) | (2,025) | (2,392)
Postretirement liability adjustment |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (33,469) | (35,604) | (33,221) | (34,257)
Other comprehensive loss before reclassifications | 0 | 0 | 0 | 0
Amounts reclassified from accumulated other comprehensive loss | 333 | 236 | 1,002 | (1,111)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  |  | (917) |
Net current-period other comprehensive income (loss) | 333 | 236 | 85 | (1,111)
Balance | (33,136) | (35,368) | (33,136) | (35,368)
Foreign currency translation adjustment |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (155) | (144) | (149) | (200)
Other comprehensive loss before reclassifications | 15 | (4) | 9 | 52
Amounts reclassified from accumulated other comprehensive loss | 0 | 0 | 0 | 0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  |  | 0 |
Net current-period other comprehensive income (loss) | 15 | (4) | 9 | 52
Balance | (140) | (148) | (140) | (148)
Net unrealized gain (loss) on available-for-sale investments |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (48) | 26 | (63) | (24)
Other comprehensive loss before reclassifications | (19) | (42) | (70) | (65)
Amounts reclassified from accumulated other comprehensive loss | 27 | 33 | 93 | 106
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  |  | 0 |
Net current-period other comprehensive income (loss) | 8 | (9) | 23 | 41
Balance | (40) | 17 | (40) | 17
Total accumulated other comprehensive loss |  |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |  |
Balance | (35,789) | (38,206) | (35,733) | (37,148)
Other comprehensive loss before reclassifications | (4) | (46) | (61) | (13)
Amounts reclassified from accumulated other comprehensive loss | 452 | 361 | 1,370 | (730)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  |  | (917) |
Net current-period other comprehensive income (loss) | 448 | 315 | 392 | (743)
Balance | $ (35,341) | $ (37,891) | $ (35,341) | $ (37,891)

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