Segment Reporting (Schedule Of Net Revenues Earned On Major Geographical Areas) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total net revenues	[1]	$ 821,572	$ 738,342	$ 2,392,779	$ 2,231,432
United States [Member]
Segment Reporting Information [Line Items]
Total net revenues		775,356	703,746	2,271,056	2,113,039
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total net revenues		43,632	32,623	113,783	109,482
Other [Member]
Segment Reporting Information [Line Items]
Total net revenues		$ 2,584	$ 1,973	$ 7,940	$ 8,911

[1] No individual client accounted for more than 10 percent of total net revenues for the three and nine months ended September 30, 2019 or 2018.