Segment Reporting (Schedule Of Net Revenues Earned On Major Geographical Areas) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total net revenues	[1]	$ 1,153,136	$ 895,817	$ 2,287,925	$ 1,808,851
United States [Member]
Segment Reporting Information [Line Items]
Total net revenues		1,058,541	810,903	2,074,436	1,671,035
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total net revenues		58,507	63,654	119,725	101,392
Canada [Member]
Segment Reporting Information [Line Items]
Total net revenues		24,656	10,082	67,012	13,625
Other [Member]
Segment Reporting Information [Line Items]
Total net revenues		$ 11,432	$ 11,178	$ 26,752	$ 22,799

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