Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (24,553)	$ (23,407)
Income taxes	(5,973)	(7,317)
Other income	(1,005)	7,595
Net Income	25,130	40,926
Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net Income	(572)	(735)
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	(147)	(148)
Income taxes	36	(249)
Net Income	(111)	(397)
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	149	100
Other income	(611)	(410)
Net Income	(462)	(310)
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	0	7
Other income	1	(35)
Net Income	$ 1	$ (28)