Segment Reporting (Schedule Of Net Revenues Earned On Major Geographical Areas) (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, 2018		Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Total net revenues	$ 793,449	$ 738,342	$ 742,732	$ 750,358	$ 804,085	$ 721,169	$ 725,647	$ 675,531	$ 3,024,881	[1]	$ 2,926,432	[1]	$ 2,575,496	[1]
United States [Member]
Segment Reporting Information [Line Items]
Total net revenues									2,855,955		2,783,175		2,429,714
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total net revenues									156,557		129,288		132,622
Other [Member]
Segment Reporting Information [Line Items]
Total net revenues									$ 12,369		$ 13,969		$ 13,160

[1] No individual client accounted for more than 10 percent of total net revenues for the years ended December 31, 2018, 2017, and 2016.