Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 594,412	$ 382,261	$ 1,402,345	$ 1,107,014
Interest expense	(6,151)	(2,989)	(13,220)	(9,900)
Interest and investment income		459	474	3,511
Income (loss) before income taxes	88,061	(191,088)	112,268	(266,296)
Provision for (benefit from) income taxes	1,082	(379)	(1,301)	206
Net income (loss)	89,143	(191,467)	110,967	(266,090)
Reclassification out of Accumulated Other Comprehensive Income | Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues		98		294
Interest expense	(336)	(638)	(1,591)	(1,502)
Provision for (benefit from) income taxes	0	0	0	0
Net income (loss)	(336)	(540)	(1,591)	(1,208)
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	(590)	(138)	(1,772)	328
Amortization of prior service credits	52	30	139	84
Pension settlement	376	437	1,364	567
Provision for (benefit from) income taxes	0	0	0	0
Net income (loss)	(162)	329	(269)	979
Reclassification out of Accumulated Other Comprehensive Income | Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and investment income	(1)	128	25	140
Provision for (benefit from) income taxes	0	0	0	0
Net income (loss)	$ (1)	$ 128	$ 25	$ 140