Comprehensive income (loss) (Details 15) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ (48,721)	$ (47,001)
Other comprehensive income (loss) before reclassifications	12,452
Amounts reclassified from accumulated other comprehensive loss	(1,936)
Net current-period other comprehensive income (loss)	10,516	(1,720)	(15,740)
Balance	(38,205)	(48,721)	(47,001)
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	6,018	6,275
Other comprehensive income (loss) before reclassifications	(5,594)
Amounts reclassified from accumulated other comprehensive loss	(4,189)
Net current-period other comprehensive income (loss)	(9,783)	(257)
Balance	(3,765)	6,018
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(54,347)	(53,320)
Other comprehensive income (loss) before reclassifications	18,539
Amounts reclassified from accumulated other comprehensive loss	2,001
Net current-period other comprehensive income (loss)	20,540	(1,027)
Balance	(33,807)	(54,347)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(511)	(38)
Other comprehensive income (loss) before reclassifications	(299)
Amounts reclassified from accumulated other comprehensive loss	143
Net current-period other comprehensive income (loss)	(156)	(473)
Balance	(667)	(511)
Net unrealized gains on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	119	82
Other comprehensive income (loss) before reclassifications	(194)
Amounts reclassified from accumulated other comprehensive loss	109
Net current-period other comprehensive income (loss)	(85)	37
Balance	$ 34	$ 119