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Components of accumulated other comprehensive loss (Details) - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Other comprehensive loss before income tax effect | $ (122,139) |  | $ (123,345)
Income tax effect | 10,312 |  | 10,695
Accumulated other comprehensive loss | (111,827) |  | (112,650)
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Equity, beginning balance | 1,017,870 |  |
Net amount reclassified to net income (loss) | 823 | $ 78 |
Equity, ending balance | 1,002,851 |  |
Defined benefit plan and other postretirement liabilities [Member] |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Other comprehensive loss before income tax effect | (120,658) |  | (121,910)
Income tax effect | 10,852 |  | 11,243
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Equity, beginning balance | (110,667) | (115,852) |
Net amount reclassified to net income (loss) | 861 | 117 |
Equity, ending balance | (109,806) | (115,735) |
Unrealized loss on financial instruments [Member] |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Other comprehensive loss before income tax effect | (1,481) |  | (1,435)
Income tax effect | (540) |  | $ (548)
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Equity, beginning balance | (1,983) | (1,830) |
Net amount reclassified to net income (loss) | (38) | (39) |
Equity, ending balance | (2,021) | (1,869) |
AOCI Attributable to Parent [Member] |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Equity, beginning balance | (112,650) | (117,682) |
Equity, ending balance | $ (111,827) | $ (117,604) |

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- Definition Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, including portion attributable to noncontrolling interest. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners). + References No definition available. + Details
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