Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 319,521	$ 570,222	$ 724,753	$ 1,125,405
Interest expense	(3,523)	(4,375)	(6,911)	(8,807)
Provision for (benefit from) income taxes	(1,206)	(91)	585	(161)
Interest and investment income	1,793	2,088	3,052	4,231
Net income (loss)	(49,324)	62,840	(74,623)	135,581
Reclassification out of Accumulated Other Comprehensive Income | Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	196	1,743	196	2,104
Interest expense	(648)	514	(864)	1,029
Provision for (benefit from) income taxes	0	0	0	0
Net income (loss)	(452)	2,257	(668)	3,133
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	232	0	466	0
Amortization of prior service credits	27		54
Pension settlement	134	429	130	429
Net income (loss)	393	429	650	429
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	141	15	140	15
Net income (loss)	$ 141	$ 15	$ 140	$ 15