Comprehensive income (loss) (Details 15) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net unrealized gain (loss) on derivative instruments qualifying as hedges	$ 6,018	$ 6,275	$ (1,625)
Net unrealized gain (loss) on derivative instruments qualifying as hedges, net of tax	(257)	7,900	673
Postretirement liability adjustment	(54,347)	(53,320)	(30,893)
Postretirement liability adjustment, net of tax	(1,027)	(22,427)	(5,730)
Foreign currency translation adjustment	(511)	(38)	1,257
Foreign currency translation adjustment, net of tax	(473)	(1,295)	(5,371)
Net unrealized gains on available-for-sale investments	119	82	0
Net unrealized gains on available-for-sale investments, net of tax	37	82	0
Accumulated other comprehensive loss	(48,721)	(47,001)	(31,261)
Other comprehensive income (loss), net of tax	$ (1,720)	$ (15,740)	$ (10,428)