Segment Reporting and Significant Concentration (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Total Revenues:
Total revenues	$ 197,288	$ 186,960	$ 182,641	$ 185,631	$ 193,697	$ 185,003	$ 184,558	$ 188,028	$ 752,520	$ 751,286	$ 747,468
Property and Equipment:
Property and Equipment	35,992				38,326				35,992	38,326
Americas (principally the U.S.)
Total Revenues:
Total revenues									634,389	636,482	633,163
Property and Equipment:
Property and Equipment	29,052				31,912				29,052	31,912
Europe, Middle East and Africa (principally Europe)
Total Revenues:
Total revenues									78,711	79,535	80,527
Property and Equipment:
Property and Equipment	3,879				3,618				3,879	3,618
Asia Pacific
Total Revenues:
Total revenues									39,420	35,269	$ 33,778
Property and Equipment:
Property and Equipment	$ 3,061				$ 2,796				$ 3,061	$ 2,796