Revenues - Revenues from Contracts with Customers Disaggregated by Service Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 448,885	$ 416,833	$ 906,771	$ 827,999
% of Revenues	100.00%	100.00%	100.00%	100.00%
Americas [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 356,427	$ 339,272	$ 720,146	$ 672,198
% of Revenues	79.40%	81.40%	79.40%	81.20%
Americas [Member] | Customer Experience Management Solutions and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 356,108	$ 338,963	$ 719,561	$ 671,577
% of Revenues	79.30%	81.30%	79.40%	81.10%
Americas [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 319	$ 309	$ 585	$ 621
% of Revenues	0.10%	0.10%	0.00%	0.10%
EMEA [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 92,455	$ 77,561	$ 186,622	$ 155,794
% of Revenues	20.60%	18.60%	20.60%	18.80%
EMEA [Member] | Customer Experience Management Solutions and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 86,950	$ 73,285	$ 176,286	$ 145,918
% of Revenues	19.40%	17.60%	19.40%	17.60%
EMEA [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 5,505	$ 4,276	$ 10,336	$ 9,876
% of Revenues	1.20%	1.00%	1.20%	1.20%
Other Segment [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 3		$ 3	$ 7
% of Revenues	0.00%	0.00%	0.00%	0.00%
Other Segment [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 3		$ 3	$ 7
% of Revenues	0.00%	0.00%	0.00%	0.00%