Comprehensive income (loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$ (5,849)	$ (3,770)
Amounts reclassified from accumulated other comprehensive gain (loss)	(2,772)	(2,329)
Net current-period other comprehensive loss	(8,621)	(6,099)
Other comprehensive income (loss) before reclassifications	(44)	(4)
Amounts reclassified from accumulated other comprehensive loss	35	30
Net current-period ither comprehensive loss	(9)	26
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	(648)	0
Net current-period other comprehensive loss	88	144
Balance at December 31, 2012	(48,721)
Other comprehensive income (loss) before reclassifications	(5,805)
Amounts reclassified from accumulated other comprehensive loss	(2,089)
Net current-period other comprehensive loss	(7,894)	(5,929)
Balance at March 31, 2013	(56,615)	(52,930)
Net unrealized gains loss on derivatives instruments qualifying as hedges [Member]
Balance at December 31, 2012	6,018
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	(5,849)
Amounts reclassified from accumulated other comprehensive gain (loss)	(2,772)
Net current-period other comprehensive loss	(8,621)
Balance at March 31, 2013	(2,603)
Net unrealized gain (loss) on available for sale investments [Member]
Balance at December 31, 2012	119
Other comprehensive income (loss) before reclassifications	(44)
Amounts reclassified from accumulated other comprehensive loss	35
Net current-period ither comprehensive loss	(9)
Balance at March 31, 2013	110
Postretirement liability adjustment [Member]
Balance at December 31, 2012	(54,347)
Other comprehensive income (loss) before reclassifications	0
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	648
Net current-period other comprehensive loss	648
Balance at March 31, 2013	(53,699)
Foreign currency translation adjustment [Member]
Balance at December 31, 2012	(511)
Other comprehensive income (loss) before reclassifications	88
Amounts reclassified from accumulated other comprehensive loss	0
Net current-period other comprehensive loss	88
Balance at March 31, 2013	$ (423)