Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (24,818)		$ (25,429)		$ (49,371)	$ (48,836)
Income taxes	(23,657)		(10,352)		(29,631)	(17,668)
Other income	10,063		1,615		9,058	9,208
Net Income	99,703	$ 25,130	61,825	$ 40,926	124,833	102,751
Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net Income	(599)		(399)		(1,171)	(1,134)
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)		(295)	(295)
Income taxes	36		36		72	(213)
Net Income	(112)		(111)		(223)	(508)
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	155		89		304	189
Other income	(635)		(365)		(1,246)	(775)
Net Income	(480)		(276)		(942)	(586)
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	2		3		2	10
Other income	(9)		(15)		(8)	(50)
Net Income	$ (7)		$ (12)		$ (6)	$ (40)