Comprehensive income (loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ (37,198)	$ (56,821)	$ (38,205)	$ (48,721)
Other comprehensive income (loss) before reclassifications	(208)	(30)	(147)	(6,142)
Amounts reclassified from accumulated other comprehensive loss	374	(86)	1,320	(2,074)
Net current-period other comprehensive income (loss)	166	(116)	1,173	(8,216)
Balance	(37,032)	(56,937)	(37,032)	(56,937)
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(3,408)	(2,744)	(3,765)	6,018
Other comprehensive income (loss) before reclassifications	0	0	0	(5,594)
Amounts reclassified from accumulated other comprehensive loss	82	(510)	439	(3,678)
Net current-period other comprehensive income (loss)	82	(510)	439	(9,272)
Balance	(3,326)	(3,254)	(3,326)	(3,254)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(33,287)	(53,275)	(33,807)	(54,347)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	261	272	781	1,344
Net current-period other comprehensive income (loss)	261	272	781	1,344
Balance	(33,026)	(53,003)	(33,026)	(53,003)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(579)	(813)	(667)	(511)
Other comprehensive income (loss) before reclassifications	(146)	(20)	(58)	(351)
Amounts reclassified from accumulated other comprehensive loss	0	114	0	143
Net current-period other comprehensive income (loss)	(146)	94	(58)	(208)
Balance	(725)	(719)	(725)	(719)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	76	11	34	119
Other comprehensive income (loss) before reclassifications	(62)	(10)	(89)	(197)
Amounts reclassified from accumulated other comprehensive loss	31	38	100	117
Net current-period other comprehensive income (loss)	(31)	28	11	(80)
Balance	$ 45	$ 39	$ 45	$ 39

- 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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