Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (23,761)			$ (25,258)			$ (73,132)	$ (74,094)
Income taxes	(31,547)			(31,098)			(61,178)	(48,766)
Other income	4,612			3,014			13,669	12,222
Net Income	153,078	$ 99,703	$ 25,130	137,637	$ 61,825	$ 40,926	277,911	240,388
Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net Income	(610)			(400)			(1,781)	(1,534)
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)			(443)	(443)
Income taxes	37			36			109	(177)
Net Income	(111)			(112)			(334)	(620)
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	152			95			456	284
Other income	(623)			(387)			(1,869)	(1,162)
Net Income	(471)			(292)			(1,413)	(878)
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	7			(1)			9	10
Other income	(35)			5			(43)	(46)
Net Income	$ (28)			$ 4			$ (34)	$ (36)