Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 570,222	$ 592,349	$ 1,125,405	$ 1,167,644
Interest expense	(4,375)	(5,050)	(8,807)	(10,445)
Provision for (benefit from) income taxes	(91)	3,366	(161)	3,910
Interest and investment income	2,088	1,552	4,231	2,825
Net income	62,840	43,306	135,581	103,291
Reclassification out of Accumulated Other Comprehensive Income | Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	1,743	0	2,104	0
Interest expense	514	348	1,029	488
Provision for (benefit from) income taxes	0	0	0	0
Net income	2,257	348	3,133	488
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
Non-service related pension and postretirement benefit (costs) credits	429	1,371	429	1,371
Net income	429	1,371	429	1,371
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	15	16	15	16
Net income	$ 15	$ 16	$ 15	$ 16