Segment and Geographical Information - Revenues and Property and Equipment, Net Based on Geographical Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues, net
Revenues, net	$ 256,872	$ 251,392	$ 243,509	$ 239,573	$ 234,903	$ 231,124	$ 210,112	$ 206,973	$ 991,346	$ 883,112	$ 762,310
Long-lived assets
Long-lived assets	165,538								165,538
Long-lived assets	79,142				73,510				79,142	73,510
India
Long-lived assets
Long-lived assets	78,244								78,244
Long-lived assets					36,152					36,152
United States
Revenues, net
Revenues, net									817,878	732,589	626,336
Long-lived assets
Long-lived assets	52,375								52,375
Long-lived assets					28,254					28,254
Philippines
Long-lived assets
Long-lived assets	26,006								26,006
Long-lived assets					5,985					5,985
United Kingdom
Revenues, net
Revenues, net									113,036	114,515	108,640
Rest of World
Revenues, net
Revenues, net									60,432	36,008	27,334
Long-lived assets
Long-lived assets	$ 8,913								8,913
Long-lived assets					$ 3,119					3,119
Total Non-United States
Revenues, net
Revenues, net									$ 173,468	$ 150,523	$ 135,974