Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									$ (96,519)	$ (98,587)	$ (84,614)
Income taxes									(84,590)	(63,279)	(47,485)
Other income (expense)									26,711	15,812	(238)
Net income	$ 112,294	$ 153,078	$ 99,703	$ 25,130	$ 95,065	$ 137,637	$ 61,825	$ 40,926	390,205	335,453	$ 272,318
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income									(2,420)	(2,257)
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									(591)	(591)
Income taxes									145	(140)
Net income									(446)	(731)
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									630	476
Other income (expense)									(2,552)	(1,962)
Net income									(1,922)	(1,486)
Reclassification adjustment for loss 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									14	10
Other income (expense)									(66)	(50)
Net income									$ (52)	$ (40)