Segment Reporting and Significant Concentration - Summary of Financial Information Relating to Operations by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Total Revenues:
Total revenues	$ 195,169	$ 189,311	$ 190,252	$ 186,226	$ 197,288	$ 186,960	$ 182,641	$ 185,631	$ 760,958	$ 752,520	$ 751,286
Property and Equipment:
Property and Equipment	33,116				35,992				33,116	35,992
Americas (principally the U.S.)
Total Revenues:
Total revenues									652,376	634,389	636,482
Property and Equipment:
Property and Equipment	28,003				29,052				28,003	29,052
Europe, Middle East and Africa (principally Europe)
Total Revenues:
Total revenues									69,425	78,711	79,535
Property and Equipment:
Property and Equipment	2,749				3,879				2,749	3,879
Asia Pacific
Total Revenues:
Total revenues									39,157	39,420	$ 35,269
Property and Equipment:
Property and Equipment	$ 2,364				$ 3,061				$ 2,364	$ 3,061