Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 549,480	$ 619,467	$ 570,222	$ 555,183	$ 650,963	$ 633,180	$ 592,349	$ 575,295	$ 2,294,352	$ 2,451,787	$ 2,324,623
Interest expense									(16,485)	(20,471)	(26,905)
Provision for (benefit from) income taxes									(248)	52,476	35,255
Total before tax									234,047	260,101	203,195
Interest and investment income									9,691	6,782	2,649
Net of tax	$ (8,551)	$ 106,769	$ 62,840	$ 72,741	$ 86,094	$ 123,192	$ 43,306	$ 59,985	233,799	312,577	$ 238,450
Pension, Postretirement and Other Post-Employment Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of net actuarial gains (losses)									3,020	0
Reclassification out of accumulated other comprehensive income | Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues									9,480	(8,206)
Interest expense									1,060	1,166
Provision for (benefit from) income taxes									0	0
Net of tax									10,540	(7,040)
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
Settlement gains									1,326	2,332
Total before tax									4,346	2,332
Net of tax									4,346	2,332
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
Interest and investment income									185	24
Net of tax									$ 185	$ 24