Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 1,467,592	$ 2,294,352	$ 2,451,787
Interest expense	(14,432)	(16,485)	(20,471)
Interest and investment income	3,808	9,691	6,782
Income (loss) before income taxes	(344,622)	234,047	260,101
Provision for (benefit from) income taxes	7	(248)	52,476
Net income (loss)	(344,615)	233,799	$ 312,577
Reclassification out of Accumulated Other Comprehensive Income | Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	392	9,480
Interest expense	(2,141)	1,060
Provision for (benefit from) income taxes	0	0
Net income (loss)	(1,749)	10,540
Reclassification out of Accumulated Other Comprehensive Income | Pension, postretirement and other post-employment benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of actuarial gains (losses), net	(191)	(3,020)
Amortization of prior service credits	112
Pension settlement	896	1,326
Income (loss) before income taxes	1,199	4,346
Provision for (benefit from) income taxes	0	0
Net income (loss)	1,199	4,346
Reclassification out of Accumulated Other Comprehensive Income | Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and investment income	265	185
Provision for (benefit from) income taxes	0	0
Net income (loss)	$ 265	$ 185