Reclassifications out of accumulated other comprehensive (loss) (Details 10) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									$ (98,587)	$ (84,614)	$ (82,788)
Income taxes									(63,279)	(47,485)	(65,041)
Other income									15,812	(238)	8,767
Net income	$ 95,065	$ 137,637	$ 61,825	$ 40,926	$ 78,828	$ 107,251	$ 43,802	$ 42,437	335,453	272,318	$ 281,203
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income									(2,257)	(2,715)
Reclassification adjustment for loss on derivative instruments included in net income (loss): | 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									(140)	429
Net income									(731)	(162)
Amortization of postretirement liability losses included in net periodic benefit cost (credit) | Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes									476	721
Other income									(1,962)	(2,894)
Net income									(1,486)	(2,173)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss) | Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes									0	75
Other income									0	(324)
Net income									0	(249)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss) | Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes									10	35
Other income									(50)	(166)
Net income									$ (40)	$ (131)