Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 382,261	$ 619,467	$ 1,107,014	$ 1,744,872
Interest expense	(2,989)	(4,049)	(9,900)	(12,856)
Provision for (benefit from) income taxes	(379)	(347)	206	(508)
Interest and investment income	459	3,709	3,511	7,940
Net income (loss)	(191,467)	106,769	(266,090)	242,350
Reclassification out of Accumulated Other Comprehensive Income | Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	98	2,963	294	5,067
Interest expense	(638)	161	(1,502)	1,190
Provision for (benefit from) income taxes	0	0	0	0
Net income (loss)	(541)	3,124	(1,209)	6,257
Reclassification out of Accumulated Other Comprehensive Income | Pension, postretirement and other post-employment benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Provision for (benefit from) income taxes	0	0	0	0
Amortization of actuarial gains (losses), net	(138)	2,236	328	2,236
Amortization of prior service credits	30		84
Pension settlement	437	197	567	626
Net income (loss)	329	2,433	979	2,862
Reclassification out of Accumulated Other Comprehensive Income | Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Provision for (benefit from) income taxes	0	0	0	0
Interest and investment income	128	35	268	50
Net income (loss)	$ 128	$ 35	$ 268	$ 50