Segment Information (Schedule Of Revenue From Unaffiliated Customers By Geographic Area) (Details) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Number of reportable segments | segment													1
Revenue from unaffiliated customers	$ 177,264	$ 152,907	$ 159,467	$ 156,431	$ 165,937	$ 156,362	$ 158,111	$ 152,555	$ 183,363	$ 151,574	$ 170,504	$ 160,722	$ 646,069	$ 632,965	$ 666,163
Operating Segments [Member] | United States [Member]
Revenue from unaffiliated customers													322,399	329,553	345,032
Operating Segments [Member] | Other Americas [Member]
Revenue from unaffiliated customers													11,377	11,332	12,944
Operating Segments [Member] | EMEA [Member]
Revenue from unaffiliated customers													220,357	193,141	200,104
Operating Segments [Member] | Asia Pacific [Member]
Revenue from unaffiliated customers													$ 91,936	$ 98,939	$ 108,083