Comprehensive income (loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Balance at December 31, 2012			$ (48,721)
Other comprehensive income (loss) before reclassifications			(6,083)
Amounts reclassified from accumulated other comprehensive loss			(2,017)
Net current-period other comprehensive income (loss)	(206)	23,355	(8,100)	17,426
Balance at June 30, 2013	(56,821)	(29,575)	(56,821)	(29,575)
Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges
Balance at December 31, 2012			6,018
Other comprehensive income (loss) before reclassifications			(5,594)
Amounts reclassified from accumulated other comprehensive loss			(3,168)
Net current-period other comprehensive income (loss)			(8,762)
Balance at June 30, 2013	(2,744)		(2,744)
Net Unrealized Gain (Loss) on Available-for-sale Investments
Balance at December 31, 2012			119
Other comprehensive income (loss) before reclassifications			(187)
Amounts reclassified from accumulated other comprehensive loss			79
Net current-period other comprehensive income (loss)			(108)
Balance at June 30, 2013	11		11
Postretirement liability adjustment
Balance at December 31, 2012			(54,347)
Other comprehensive income (loss) before reclassifications			0
Amounts reclassified from accumulated other comprehensive loss			1,072
Net current-period other comprehensive income (loss)			1,072
Balance at June 30, 2013	(53,275)		(53,275)
Foreign Currency Translation Adjustment
Balance at December 31, 2012			(511)
Other comprehensive income (loss) before reclassifications			(302)
Amounts reclassified from accumulated other comprehensive loss			0
Net current-period other comprehensive income (loss)			(302)
Balance at June 30, 2013	$ (813)		$ (813)