Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 555,183	$ 575,295
Interest expense	(4,432)	(5,395)
Interest and investment income	2,143	1,273
Provision for (benefit from) income taxes	70	(544)
Net income	72,741	59,985
Reclassification out of Accumulated Other Comprehensive Income | Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	361	0
Interest expense	515	140
Provision for (benefit from) income taxes	0	0
Net income	876	140
Reclassification out of Accumulated Other Comprehensive Income | Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and investment income	0	0
Provision for (benefit from) income taxes	0	0
Net income	$ 0	$ 0