Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ (40,924)	$ (37,538)	$ (42,103)	$ (38,205)
Other comprehensive loss before reclassifications	(34)	50	(167)	98
Amounts reclassified from accumulated other comprehensive loss	712	290	2,024	909
Net current-period other comprehensive income (loss)	678	340	1,857	1,007
Balance	(40,246)	(37,198)	(40,246)	(37,198)
Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,972)	(3,421)	(3,071)	(3,765)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	100	13	199	357
Net current-period other comprehensive income (loss)	100	13	199	357
Balance	(2,872)	(3,408)	(2,872)	(3,408)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,843)	(33,532)	(38,218)	(33,807)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	584	245	959	520
Net current-period other comprehensive income (loss)	584	245	959	520
Balance	(37,259)	(33,287)	(37,259)	(33,287)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(139)	(621)	(829)	(667)
Other comprehensive loss before reclassifications	9	42	(103)	88
Amounts reclassified from accumulated other comprehensive loss	0	0	802	0
Net current-period other comprehensive income (loss)	9	42	699	88
Balance	(130)	(579)	(130)	(579)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	30	36	15	34
Other comprehensive loss before reclassifications	(43)	8	(64)	10
Amounts reclassified from accumulated other comprehensive loss	28	32	64	32
Net current-period other comprehensive income (loss)	(15)	40	0	42
Balance	$ 15	$ 76	$ 15	$ 76