Supplemental financial statement information - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive loss before income tax effect	$ (123,345)	$ (129,913)
Income tax effect	10,695	12,231
Accumulated other comprehensive loss	(112,650)	(117,682)	$ (91,832)	$ (151,192)
Defined benefit plan liabilities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive loss before income tax effect	(121,910)	(128,664)
Income tax effect	11,243	12,812
Accumulated other comprehensive loss	(110,667)	(115,852)	(77,921)	(137,441)
Unrealized loss on financial instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive loss before income tax effect	(1,435)	(1,249)
Income tax effect	(548)	(581)
Accumulated other comprehensive loss	$ (1,983)	$ (1,830)	$ (1,679)	$ (1,527)

- Definition

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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