Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (25,258)			$ (20,955)			$ (74,094)	$ (62,202)
Income taxes	(31,098)			(14,363)			(48,766)	(32,629)
Other income	3,014			2,683			12,222	4,864
Net Income	137,637	$ 61,825	$ 40,926	107,251	$ 43,802	$ 42,437	240,388	193,490
Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net Income	(400)			(567)			(1,534)	(1,934)
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)			(147)			(443)	(443)
Income taxes	36			55			(177)	164
Net Income	(112)			(92)			(620)	(279)
Amortization of postretirement liability losses included in net periodic benefit cost | Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	95			142			284	442
Other income	(387)			(584)			(1,162)	(1,751)
Net Income	(292)			(442)			(878)	(1,309)
Reclassification adjustment for 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]
Income taxes	0			0			0	75
Other income	0			0			0	(324)
Net Income	0			0			0	(249)
Reclassification adjustment 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]
Income taxes	(1)			9			10	26
Other income	5			(42)			(46)	(123)
Net Income	$ 4			$ (33)			$ (36)	$ (97)