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Comprehensive income (loss) (Details 9) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |
Balance | $ 2,429,043 | $ 2,316,244 | $ 2,520,548
Net current-period other comprehensive income (loss) | 6,951 | (1,601) | 1,415
Balance | 2,566,775 | 2,429,043 | 2,316,244
Net unrealized gain (loss) on derivative instruments qualifying as hedges |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |
Balance | (1,934) | (2,300) |
Other comprehensive income (loss) before reclassifications | 0 | 0 |
Amounts reclassified from accumulated other comprehensive loss | 162 | 366 |
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | 0 |
Net current-period other comprehensive income (loss) | 162 | 366 |
Balance | (2,161) | (1,934) | (2,300)
Postretirement liability adjustment |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |
Balance | (35,163) | (33,221) |
Other comprehensive income (loss) before reclassifications | 4,441 | (1,812) |
Amounts reclassified from accumulated other comprehensive loss | 2,173 | 1,013 |
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | (1,143) |
Net current-period other comprehensive income (loss) | 6,614 | (1,942) |
Balance | (36,069) | (35,163) | (33,221)
Foreign currency translation adjustment |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |
Balance | (155) | (149) |
Other comprehensive income (loss) before reclassifications | (61) | (6) |
Amounts reclassified from accumulated other comprehensive loss | 249 | 0 |
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | 0 |
Net current-period other comprehensive income (loss) | 188 | (6) |
Balance | 0 | (155) | (149)
Net unrealizaed gasin (loss) on available-for-sale investments |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |
Balance | (82) | (63) |
Other comprehensive income (loss) before reclassifications | (144) | (139) |
Amounts reclassified from accumulated other comprehensive loss | 131 | 120 |
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | 0 |
Net current-period other comprehensive income (loss) | (13) | (19) |
Balance | (112) | (82) | (63)
Accumulated other comprehensive loss |  |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |  |
Balance | (37,334) | (35,733) | (37,148)
Other comprehensive income (loss) before reclassifications | 4,236 | (1,957) |
Amounts reclassified from accumulated other comprehensive loss | 2,715 | 1,499 |
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | (1,143) |
Net current-period other comprehensive income (loss) | 6,951 | (1,601) | 1,415
Balance | (38,342) | $ (37,334) | $ (35,733)
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 unrealizaed gasin (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 | 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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- Definition Amount after tax, before reclassification adjustments of other comprehensive income (loss). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669686-108580 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 55 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=109228884&loc=d3e1436-108581 + Details
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