Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 650,963	$ 633,180	$ 592,349	$ 575,295	$ 560,244	$ 613,538	$ 549,866	$ 600,975	$ 575,688	$ 2,451,787	$ 2,324,623
Interest expense									(11,241)	(20,471)	(26,905)
Provision for (benefit from) income taxes									(1,156)	52,476	35,255
Total before tax									34,605	260,101	203,195
Interest and investment income									487	6,782	2,649
Net of tax	$ 86,094	$ 123,192	$ 43,306	$ 59,985	$ 81,271	$ 68,351	$ 37,160	$ 51,668	$ 33,449	312,577	238,450
Reclassification out of accumulated other comprehensive income | Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues										(8,206)	0
Interest expense										1,166	(150)
Provision for (benefit from) income taxes										0	0
Net of tax										(7,040)	(150)
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
Curtailments										0	(773)
Settlement gains										2,332	1,531
Total before tax										2,332	758
Net of tax										2,332	758
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										24	387
Net of tax										$ 24	$ 387