Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (20,800)	$ (20,766)		$ (41,246)	$ (41,068)
Other income	1,599	1,801		2,181	4,144
Income taxes	(10,716)	(15,290)	[1]	(18,267)	(27,478)	[1]
Net income	43,802	41,215		86,239	78,539
Total reclassifications	(827)	(435)		(1,367)	(918)
Reclassification adjustment for loss on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Total reclassifications	95	92		187	183
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	(148)	(148)		(296)	(295)
Income taxes	53	56		109	112
Net income	(95)	(92)		(187)	(183)
Amortization of postretirement liability losses included in net periodic benefit cost (credit)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Total reclassifications	449	312		867	669
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	(594)	(502)		(1,167)	(1,075)
Income taxes	145	190		300	406
Net income	(449)	(312)		(867)	(669)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Total reclassifications	249	0		249	0
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	(324)	0		(324)	0
Income taxes	75	0		75	0
Net income	(249)	0		(249)	0
Reclassification adjustment for loss on available-for-sale investments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Total reclassifications	34	31		64	66
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	(44)	(48)		(81)	(102)
Income taxes	10	17		17	36
Net income	$ (34)	$ (31)		$ (64)	$ (66)

[1] Includes the eliminations for the presentation of income tax adjustments between continuing and discontinued operations.