Segment Reporting (Schedule Of Net Revenues Earned On Major Geographical Areas) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total net revenues	[1]	$ 883,300	$ 821,572	$ 2,692,151	$ 2,392,779
United States [Member]
Segment Reporting Information [Line Items]
Total net revenues		814,301	775,356	2,485,336	2,271,056
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total net revenues		45,705	43,632	147,097	113,783
Other [Member]
Segment Reporting Information [Line Items]
Total net revenues		$ 23,294	$ 2,584	$ 59,718	$ 7,940

[1] No individual client accounted for more than 10 percent of total net revenues for the three and nine months ended September 30, 2020 or 2019.