Segment Reporting (Schedule Of Net Revenues Earned On Major Geographical Areas) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Segment Reporting Information [Line Items]
Total net revenues	$ 1,059,910	$ 883,300	$ 895,817	$ 913,034	$ 944,251	$ 821,572	$ 800,787	$ 770,420	$ 3,752,061	[1]	$ 3,337,030	[1]	$ 3,024,881	[1]
United States [Member]
Segment Reporting Information [Line Items]
Total net revenues									3,460,930		3,154,285		2,855,955
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total net revenues									197,859		163,552		156,557
Other [Member]
Segment Reporting Information [Line Items]
Total net revenues									$ 93,272		$ 19,193		$ 12,369

[1] No individual client accounted for more than 10 percent of total net revenues for the years ended December 31, 2020, 2019, and 2018.