Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (22,219)	$ (23,389)	$ (45,087)	$ (46,456)
Income taxes	(21,320)	(12,382)	(29,620)	(19,333)
Other income	872	2,123	1,921	2,373
Net loss	(229,804)	(237,358)	(215,974)	(546,803)
Reclassification from accumulated other comprehensive income, current period, net of tax	(375)	(712)	1,095	(2,024)
Reclassification adjustment for loss on derivative instruments included in net loss:
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	91	100	183	199
Reclassification adjustment for loss on derivative instruments included in net 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	(147)	(160)	(297)	(320)
Income taxes	56	60	114	121
Net loss	(91)	(100)	(183)	(199)
Amortization of postretirement liability gains (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	248	584	(1,347)	959
Amortization of postretirement liability gains (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	150	420	(819)	649
Net periodic benefit cost	(398)	(1,004)	2,166	(1,608)
Net loss	(248)	(584)	1,347	(959)
Reclassification adjustment for loss on foreign currency translation adjustment included in net loss
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	0	0	0	802
Reclassification adjustment for loss on foreign currency translation adjustment included in net loss | Reclassification out of accumulated other comprehensive income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	0	0	0	491
Other income	0	0	0	(1,293)
Net loss	0	0	0	(802)
Reclassification adjustment for loss on available-for-sale investments included in net loss
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	36	28	69	64
Reclassification adjustment for loss on available-for-sale investments included in net loss | Reclassification out of accumulated other comprehensive income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	19	15	37	34
Other income	(55)	(43)	(106)	(98)
Net loss	$ (36)	$ (28)	$ (69)	$ (64)