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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total Revenues:
Total revenues	$ 205,204	$ 199,195	$ 192,713	$ 192,470	$ 195,169	$ 189,311	$ 190,252	$ 186,226	$ 789,582	$ 760,958	$ 752,520
Property and Equipment:
Property and Equipment	44,651				33,116				44,651	33,116
Americas (principally the U.S.)
Total Revenues:
Total revenues									669,712	652,376	634,389
Property and Equipment:
Property and Equipment	40,286				28,003				40,286	28,003
Europe, Middle East and Africa (principally Europe)
Total Revenues:
Total revenues									72,120	69,425	78,711
Property and Equipment:
Property and Equipment	2,392				2,749				2,392	2,749
Asia Pacific
Total Revenues:
Total revenues									47,750	39,157	$ 39,420
Property and Equipment:
Property and Equipment	$ 1,973				$ 2,364				$ 1,973	$ 2,364