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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total Revenues:
Total revenues	$ 247,267	$ 213,055	$ 213,033	$ 201,704	$ 205,204	$ 199,195	$ 192,713	$ 192,470	$ 875,059	$ 789,582	$ 760,958
Property and Equipment:
Property and Equipment	81,813				44,651				81,813	44,651
Americas (principally the U.S.)
Total Revenues:
Total revenues									740,885	669,712	652,376
Property and Equipment:
Property and Equipment	72,079				40,286				72,079	40,286
Europe, Middle East and Africa (principally Europe)
Total Revenues:
Total revenues									89,924	72,120	69,425
Property and Equipment:
Property and Equipment	2,056				2,392				2,056	2,392
Asia Pacific
Total Revenues:
Total revenues									44,250	47,750	$ 39,157
Property and Equipment:
Property and Equipment	$ 7,678				$ 1,973				$ 7,678	$ 1,973