Segment Reporting and Significant Concentration (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Total Revenues:
Total revenues	$ 198,007	$ 190,001	$ 183,851	$ 185,007	$ 187,574	$ 182,753	$ 181,312	$ 183,092	$ 756,866	$ 734,731	$ 549,379
Property and Equipment:
Property and Equipment	39,429				41,154				39,429	41,154
Americas (principally the U.S.) [Member]
Total Revenues:
Total revenues									652,008	627,231	540,377
Property and Equipment:
Property and Equipment	34,796				36,149				34,796	36,149
Europe, Middle East and Africa (principally Europe) [Member]
Total Revenues:
Total revenues									73,113	75,938	7,482
Property and Equipment:
Property and Equipment	2,238				2,159				2,238	2,159
Asia Pacific [Member]
Total Revenues:
Total revenues									31,745	31,562	1,520
Property and Equipment:
Property and Equipment	$ 2,395				$ 2,846				$ 2,395	$ 2,846