Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 575,295	$ 600,975
Interest expense	(5,395)	(9,425)
Provision for (benefit from) income taxes	544	(840)
Net income	59,985	51,668
Net of tax	140
Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net of tax	140
Pension, Postretirement and Other Postemployment Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	0	0
Provision for (benefit from) income taxes	0	0
Net of tax	0	0
Amortization of prior service credits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	0	0
Amortization of actuarial gains (losses), net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	0
Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net of tax	0
Reclassification out of Accumulated Other Comprehensive Income | Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	0	0
Interest expense	140	0
Provision for (benefit from) income taxes	0	0
Net income	140	0
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	0	332
Net income	$ 0	$ 332