Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (20,766)	$ (22,219)	$ (41,068)	$ (45,087)
Income taxes	(15,290)	(21,320)	(27,478)	(29,620)
Other income	782	872	1,798	1,921
Net income (loss)	41,215	(229,804)	78,539	(215,974)
Reclassification from accumulated other comprehensive income, current period, net of tax	(435)	(375)	(918)	1,095
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	91	183	183
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	(148)	(147)	(295)	(297)
Income taxes	56	56	112	114
Net income (loss)	(92)	(91)	(183)	(183)
Amortization of postretirement liability gains (losses) included in net periodic benefit cost (credit)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	312	248	669	(1,347)
Amortization of postretirement liability gains (losses) included in net periodic benefit cost (credit) | Reclassification out of accumulated other comprehensive income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	190	150	406	(819)
Net periodic benefit cost (credit)	(502)	(398)	(1,075)	2,166
Net income (loss)	(312)	(248)	(669)	1,347
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	31	36	66	69
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	17	19	36	37
Other income	(48)	(55)	(102)	(106)
Net income (loss)	$ (31)	$ (36)	$ (66)	$ (69)