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Supplemental financial statement information - Changes in components of Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Beginning Balance |  |  |  | $ (117,682) |  |  |  | $ (91,832) | $ (117,682) | $ (91,832) | $ (151,192)
Other comprehensive income (loss) before reclassifications |  |  |  |  |  |  |  |  | 7,311 | (43,345) | 56,787
Elimination of equity in investee resulting from acquisition |  |  |  |  |  |  |  |  |  | 12,651 |
Net amount reclassified to net income |  |  |  |  |  |  |  |  | (2,279) | 4,844 | 2,573
Ending Balance | $ (112,650) |  |  |  | $ (117,682) |  |  |  | (112,650) | (117,682) | (91,832)
Cost of goods sold |  |  |  |  |  |  |  |  | 1,908,544 | 1,729,243 | 1,414,790
Selling, general and administrative expenses |  |  |  |  |  |  |  |  | 42,115 | 49,195 | 67,477
Other operating expense, net |  |  |  |  |  |  |  |  | (356) | 991 | 496
Income tax expense |  |  |  |  |  |  |  |  | 9,276 | 18,308 | 3,131
Net income (loss) | (43,080) | $ (56,112) | $ (33,897) | 73,779 | 75,829 | $ 50,405 | $ 20,344 | (20,104) | (59,310) | 126,474 | (40,313)
Defined benefit plan liabilities [Member] |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Beginning Balance |  |  |  | (115,852) |  |  |  | (77,921) | (115,852) | (77,921) | (137,441)
Other comprehensive income (loss) before reclassifications |  |  |  |  |  |  |  |  | 7,311 | (42,926) | 56,795
Elimination of equity in investee resulting from acquisition |  |  |  |  |  |  |  |  |  | 0 |
Net amount reclassified to net income |  |  |  |  |  |  |  |  | (2,126) | 4,995 | 2,725
Ending Balance | (110,667) |  |  |  | (115,852) |  |  |  | (110,667) | (115,852) | (77,921)
Equity in investee other comprehensive income [Member] |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Beginning Balance |  |  |  | 0 |  |  |  | (12,232) | 0 | (12,232) | (12,224)
Other comprehensive income (loss) before reclassifications |  |  |  |  |  |  |  |  | 0 | (419) | (8)
Elimination of equity in investee resulting from acquisition |  |  |  |  |  |  |  |  |  | 12,651 |
Net amount reclassified to net income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Ending Balance | 0 |  |  |  | 0 |  |  |  | 0 | 0 | (12,232)
Unrealized loss on financial instruments [Member] |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Beginning Balance |  |  |  | $ (1,830) |  |  |  | $ (1,679) | (1,830) | (1,679) | (1,527)
Other comprehensive income (loss) before reclassifications |  |  |  |  |  |  |  |  | 0 | 0 | 0
Elimination of equity in investee resulting from acquisition |  |  |  |  |  |  |  |  |  | 0 |
Net amount reclassified to net income |  |  |  |  |  |  |  |  | (153) | (151) | (152)
Ending Balance | $ (1,983) |  |  |  | $ (1,830) |  |  |  | (1,983) | (1,830) | (1,679)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined benefit plan liabilities [Member] |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Cost of goods sold |  |  |  |  |  |  |  |  | (1,696) | 6,237 | 3,264
Selling, general and administrative expenses |  |  |  |  |  |  |  |  | 235 | 569 | 990
Other operating expense, net |  |  |  |  |  |  |  |  | 904 | 0 | 0
Income tax expense |  |  |  |  |  |  |  |  | (1,569) | (1,811) | (1,529)
Net income (loss) |  |  |  |  |  |  |  |  | (2,126) | 4,995 | 2,725
Reclassification out of Accumulated Other Comprehensive Income [Member] | Equity in investee other comprehensive income [Member] |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Cost of goods sold |  |  |  |  |  |  |  |  | 0 | 0 | 0
Income tax expense |  |  |  |  |  |  |  |  | 0 | (419) | (70)
Net income (loss) |  |  |  |  |  |  |  |  | 0 | (419) | (70)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized loss on financial instruments [Member] |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Cost of goods sold |  |  |  |  |  |  |  |  | (186) | (185) | (186)
Income tax expense |  |  |  |  |  |  |  |  | 33 | 34 | 34
Net income (loss) |  |  |  |  |  |  |  |  | $ (153) | $ (151) | $ (152)

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