Segment Reporting (Schedule Of Net Revenues Earned On Major Geographical Areas) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Total net revenues	[1]	$ 1,144,938	$ 883,300	$ 3,432,863	$ 2,692,151
United States [Member]
Segment Reporting Information [Line Items]
Total net revenues		1,067,901	814,301	3,142,337	2,485,336
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total net revenues		57,413	45,705	177,138	147,097
Canada [Member]
Segment Reporting Information [Line Items]
Total net revenues		9,523	10,713	76,535	24,338
Other [Member]
Segment Reporting Information [Line Items]
Total net revenues		$ 10,101	$ 12,581	$ 36,853	$ 35,380

[1] No individual client accounted for more than 10 percent of total net revenues for the three and nine months ended September 30, 2021 or 2020.