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Comprehensive income (loss) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | $ 2,429,043 | $ 2,316,244
Amounts reclassified from accumulated other comprehensive loss | (540) | (483)
Net current-period other comprehensive income (loss) | 433 | (452)
Balance | 2,426,111 | 2,313,905
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 | 92 | 91
Net current-period other comprehensive income (loss) | 92 | 91
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | 0
Balance | (2,231) | (2,209)
Postretirement liability adjustment |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | (35,163) | (33,221)
Other comprehensive income (loss) before reclassifications | 0 | 0
Amounts reclassified from accumulated other comprehensive loss | 418 | 357
Net current-period other comprehensive income (loss) | 418 | (560)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | (917)
Balance | (42,265) | (33,781)
Foreign currency translation adjustment |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | (155) | (149)
Other comprehensive income (loss) before reclassifications | (2) | 9
Amounts reclassified from accumulated other comprehensive loss | 0 | 0
Net current-period other comprehensive income (loss) | (2) | 9
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | 0
Balance | (190) | (140)
Net unrealized gain (loss) on available-for-sale investments |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | (82) | (63)
Other comprehensive income (loss) before reclassifications | (105) | (27)
Amounts reclassified from accumulated other comprehensive loss | 30 | 35
Net current-period other comprehensive income (loss) | (75) | 8
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | 0
Balance | (174) | (55)
Total accumulated other comprehensive loss |  |
Accumulated other comprehensive income (loss) [Roll Forward] |  |
Balance | (37,334) | (35,733)
Other comprehensive income (loss) before reclassifications | (107) | (18)
Amounts reclassified from accumulated other comprehensive loss | 540 | 483
Net current-period other comprehensive income (loss) | 433 | (452)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset |  | (917)
Balance | (44,860) | $ (36,185)
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) |
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) |
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) |
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) |
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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- Definition Amount after tax, before reclassification adjustments of other comprehensive income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 55 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=84158357&loc=d3e1436-108581 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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