Segment Reporting (Schedule Of Net Revenues Earned On Major Geographical Areas) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total net revenues	[1]	$ 738,342	$ 721,169	$ 2,231,432	$ 2,122,347
United States [Member]
Segment Reporting Information [Line Items]
Total net revenues		703,746	680,505	2,113,039	2,017,999
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total net revenues		32,623	38,103	109,482	95,393
Other [Member]
Segment Reporting Information [Line Items]
Total net revenues		$ 1,973	$ 2,561	$ 8,911	$ 8,955

[1] No individual client accounted for more than 10 percent of total net revenues for the three and nine months ended September 30, 2018 or 2017.