Comprehensive income (loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ (42,103)	$ (38,205)
Other comprehensive income (loss) before reclassifications	(133)	10
Amounts reclassified from accumulated other comprehensive loss	1,312	657
Net current-period other comprehensive income (loss)	1,179	667
Balance	(40,924)	(37,538)
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(3,071)	(3,765)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	99	344
Net current-period other comprehensive income (loss)	99	344
Balance	(2,972)	(3,421)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(38,218)	(33,807)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	375	275
Net current-period other comprehensive income (loss)	375	275
Balance	(37,843)	(33,532)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(829)	(667)
Other comprehensive income (loss) before reclassifications	(112)	46
Amounts reclassified from accumulated other comprehensive loss	802	0
Net current-period other comprehensive income (loss)	690	46
Balance	(139)	(621)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	15	34
Other comprehensive income (loss) before reclassifications	(21)	(36)
Amounts reclassified from accumulated other comprehensive loss	36	38
Net current-period other comprehensive income (loss)	15	2
Balance	$ 30	$ 36