Segment Reporting (Schedule Of Net Revenues Earned On Major Geographical Areas) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total net revenues	[1]	$ 895,817	$ 800,787	$ 1,808,851	$ 1,571,207
United States [Member]
Segment Reporting Information [Line Items]
Total net revenues		810,903	754,470	1,671,035	1,495,700
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total net revenues		63,654	43,761	101,392	70,151
Other [Member]
Segment Reporting Information [Line Items]
Total net revenues		$ 21,260	$ 2,556	$ 36,424	$ 5,356

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