Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (23,776)	$ (23,127)
Income taxes	(4,558)	(5,825)
Other income	1,246	443
Net income (loss)	13,829	(309,445)
Reclassification from accumulated other comprehensive income, current period, net of tax	1,467	(1,312)
Reclassification adjustment for loss on derivative instruments included in net income (loss):
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	92	99
Reclassification adjustment for loss on derivative instruments included in net income (loss): | Reclassification out of accumulated other comprehensive income [Member] | Interest rate contract [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	(149)	(159)
Income taxes	57	60
Net income (loss)	(92)	(99)
Amortization of postretirement liability losses included in net periodic benefit cost
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	(1,595)	375
Amortization of postretirement liability losses included in net periodic benefit cost | Reclassification out of accumulated other comprehensive income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	(969)	230
Net periodic benefit cost	2,564	(605)
Net income (loss)	1,595	(375)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	0	802
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss) | Reclassification out of accumulated other comprehensive income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	0	490
Other income	0	(1,292)
Net income (loss)	0	(802)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	36	36
Reclassification adjustment for loss on available-for-sale investments included in net income (loss) | Reclassification out of accumulated other comprehensive income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	19	19
Other income	(55)	(55)
Net income (loss)	$ (36)	$ (36)