Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (23,790)	$ (21,484)	$ (46,919)	$ (42,431)
Income (loss) from discontinued operations, net of tax	(251,415)	23,881	(576,020)	48,993
Other income	2,320	2,470	2,764	4,638
Income taxes	(9,801)	(13,894)	(15,626)	(27,696)
Income from continuing operations	14,057	29,446	29,217	60,473
Net income (loss)	(237,358)	53,327	(546,803)	109,466
Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	60	60	121	121
Income from continuing operations	(100)	(100)	(199)	(199)
Net income (loss)	(100)	(13)	(199)	(357)
Postretirement Liability Adjustment
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net periodic benefit cost	(1,004)	(395)	(1,608)	(838)
Income taxes	420	150	649	318
Net income (loss)	(584)	(245)	(959)	(520)
Foreign Currency Translation Adjustment
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	0	0	(1,293)	0
Income taxes	0	0	491	0
Net income (loss)	0	0	(802)	0
Net Unrealized Gain on Available-for-sale Investments
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	(43)	(49)	(98)	(49)
Income taxes	15	17	34	17
Net income (loss)	(28)	(32)	(64)	(32)
Total Accumulated Other Comprehensive Loss
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net income (loss)	(712)	(290)	(2,024)	(909)
Interest rate derivative instruments [Member] | Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	(160)	(160)	(320)	(320)
Commodity derivative instruments [Member] | Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income (loss) from discontinued operations, net of tax	$ 0	$ 87	$ 0	$ (158)