Reclassification out of accumulated other comprehensive loss (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Interest expense	$ (23,453)	$ (24,553)
Income taxes	(12,949)	(5,973)
Other income	3,354	(1,005)
Net Income	52,131	25,130
Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Net Income	(612)	(572)
Reclassification adjustment for loss on derivative instruments included in net income | Reclassification out of accumulated other comprehensive loss | Interest rate contract
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Interest expense	(148)	(147)
Income taxes	37	36
Net Income	(111)	(111)
Amortization of postretirement liability losses included in net periodic benefit cost | Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Income taxes	151	149
Other income	(617)	(611)
Net Income	(466)	(462)
Reclassification adjustment on available-for-sale investments included in net income | Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Income taxes	9	0
Other income	(44)	1
Net Income	$ (35)	$ 1