Comprehensive income (loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ (37,538)	$ (56,615)	$ (38,205)	$ (48,721)
Other comprehensive income (loss) before reclassifications	50	(278)	98	(6,083)
Amounts reclassified from accumulated other comprehensive loss	290	72	909	(2,017)
Net current-period other comprehensive income (loss)	340	(206)	1,007	(8,100)
Balance	(37,198)	(56,821)	(37,198)	(56,821)
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(3,421)	(2,603)	(3,765)	6,018
Other comprehensive income (loss) before reclassifications	0	254	0	(5,594)
Amounts reclassified from accumulated other comprehensive loss	13	(395)	357	(3,168)
Net current-period other comprehensive income (loss)	13	(141)	357	(8,762)
Balance	(3,408)	(2,744)	(3,408)	(2,744)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(33,532)	(53,699)	(33,807)	(54,347)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	245	424	520	1,072
Net current-period other comprehensive income (loss)	245	424	520	1,072
Balance	(33,287)	(53,275)	(33,287)	(53,275)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(621)	(423)	(667)	(511)
Other comprehensive income (loss) before reclassifications	42	(390)	88	(302)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive income (loss)	42	(390)	88	(302)
Balance	(579)	(813)	(579)	(813)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	36	110	34	119
Other comprehensive income (loss) before reclassifications	8	(142)	10	(187)
Amounts reclassified from accumulated other comprehensive loss	32	43	32	79
Net current-period other comprehensive income (loss)	40	(99)	42	(108)
Balance	$ 76	$ 11	$ 76	$ 11