Revenues - Revenues from Contracts with Customers Disaggregated by Service Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 1,614,762	$ 1,625,687	$ 1,586,008
% of Revenues	100.00%	100.00%	100.00%
Customer Engagement Solutions and Services [Member]
Disaggregation of Revenue [Line Items]
% of Revenues	97.70%	99.00%	99.40%
Americas [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 1,296,660	$ 1,330,638	$ 1,325,643
% of Revenues	80.30%	81.90%	83.60%
Americas [Member] | Customer Engagement Solutions and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 1,295,636	$ 1,329,614	$ 1,324,534
% of Revenues	80.30%	81.80%	83.50%
Americas [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 1,024	$ 1,024	$ 1,109
% of Revenues	0.00%	0.10%	0.10%
EMEA [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 318,013	$ 294,954	$ 260,283
% of Revenues	19.70%	18.10%	16.40%
EMEA [Member] | Customer Engagement Solutions and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 281,302	$ 280,437	$ 252,423
% of Revenues	17.40%	17.20%	15.90%
EMEA [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 36,711	$ 14,517	$ 7,860
% of Revenues	2.30%	0.90%	0.50%
Other Segment [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 89	$ 95	$ 82
% of Revenues	0.00%	0.00%	0.00%
Other Segment [Member] | Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 89	$ 95	$ 82
% of Revenues	0.00%	0.00%	0.00%