Comprehensive income (loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Balance at December 31, 2012			$ (48,721)
Other comprehensive income (loss) before reclassifications			(6,142)
Amounts reclassified from accumulated other comprehensive loss			(2,074)
Net current-period other comprehensive income (loss)	(116)	(16,873)	(8,216)	553
Balance at September 30, 2013	(56,937)	(46,448)	(56,937)	(46,448)
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			(3,678)
Net current-period other comprehensive income (loss)			(9,272)
Balance at September 30, 2013	(3,254)		(3,254)
Net Unrealized Gain (Loss) on Available-for-sale Investments
Balance at December 31, 2012			119
Other comprehensive income (loss) before reclassifications			(197)
Amounts reclassified from accumulated other comprehensive loss			117
Net current-period other comprehensive income (loss)			(80)
Balance at September 30, 2013	39		39
Postretirement liability adjustment
Balance at December 31, 2012			(54,347)
Other comprehensive income (loss) before reclassifications			0
Amounts reclassified from accumulated other comprehensive loss			1,344
Net current-period other comprehensive income (loss)			1,344
Balance at September 30, 2013	(53,003)		(53,003)
Foreign Currency Translation Adjustment
Balance at December 31, 2012			(511)
Other comprehensive income (loss) before reclassifications			(351)
Amounts reclassified from accumulated other comprehensive loss			143
Net current-period other comprehensive income (loss)			(208)
Balance at September 30, 2013	$ (719)		$ (719)