Segment Reporting and Significant Concentration (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Total Revenues:
Total revenues	$ 193,697	$ 185,003	$ 184,558	$ 188,028	$ 194,549	$ 186,180	$ 186,107	$ 180,632	$ 751,286	$ 747,468	$ 756,866
Property and Equipment:
Property and Equipment	38,326				35,061				38,326	35,061
Americas (principally the U.S.)
Total Revenues:
Total revenues									636,482	633,163	652,008
Property and Equipment:
Property and Equipment	31,912				27,115				31,912	27,115
Europe, Middle East and Africa (principally Europe)
Total Revenues:
Total revenues									79,535	80,527	73,113
Property and Equipment:
Property and Equipment	3,618				4,280				3,618	4,280
Asia Pacific
Total Revenues:
Total revenues									35,269	33,778	31,745
Property and Equipment:
Property and Equipment	$ 2,796				$ 3,666				$ 2,796	$ 3,666