Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 592,349	$ 549,866	$ 1,167,644	$ 1,150,841
Interest expense	(5,050)	(6,003)	(10,445)	(15,428)
Pension settlement	68	(232)	(1,235)	(953)
Provision for (benefit from) income taxes	3,366	(319)	3,910	(1,159)
Net income	43,306	37,160	103,291	88,828
Reclassification out of Accumulated Other Comprehensive Income | Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	0	0	0	0
Interest expense	348	0	488	0
Provision for (benefit from) income taxes	0	0	0	0
Net income	348	0	488	0
Reclassification out of Accumulated Other Comprehensive Income | Pension, Postretirement and Other Postemployment Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension settlement	1,371	487	1,371	487
Provision for (benefit from) income taxes	0	0	0	0
Net income	1,371	487	1,371	487
Reclassification out of Accumulated Other Comprehensive Income | Pension settlement
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension settlement	1,371	487	1,371	487
Reclassification out of Accumulated Other Comprehensive Income | Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and investment income	16	0	16	332
Provision for (benefit from) income taxes	0	0	0	0
Net income	$ 16	$ 0	$ 16	$ 332