Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (20,955)	$ (20,909)	$ (62,202)	$ (61,978)
Other income	2,683	2,068	4,864	6,212
Income taxes	(14,363)	(46,930)	(32,629)	(74,406)
Total reclassifications	107,251	87,351	193,490	165,891
Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Total reclassifications	(567)	(452)	(1,934)	(1,370)
Reclassification adjustment for loss on derivative instruments included in net income | Reclassification out of accumulated other comprehensive income | Interest rate contract
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	(147)	(148)	(443)	(443)
Income taxes	55	56	164	168
Total reclassifications	(92)	(92)	(279)	(275)
Amortization of postretirement liability losses included in net periodic benefit cost (credit) | Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	(584)	(536)	(1,751)	(1,611)
Income taxes	142	203	442	609
Total reclassifications	(442)	(333)	(1,309)	(1,002)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income | Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	0	0	(324)	0
Income taxes	0	0	75	0
Total reclassifications	0	0	(249)	0
Reclassification adjustment for loss on available-for-sale investments included in net income | Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	(42)	(41)	(123)	(143)
Income taxes	9	14	26	50
Total reclassifications	$ (33)	$ (27)	$ (97)	$ (93)