ACQUISITIONS AND DIVESTITURES (NARRATIVE) (Details) £ in Millions								3 Months Ended		12 Months Ended
	Feb. 07, 2020 USD ($)	Oct. 26, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 30, 2018 GBP (£)	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Other liabilities, noncurrent				$ 126,532,000						$ 79,641,000	$ 126,532,000
Contingent Consideration, at fair value				2,363,000						6,134,000	2,363,000		$ 399,000
Revenue of Acquirees since Acquisition Date, Actual										21,500,000	3,100,000
Income (loss) from operations of Acquirees since Acquisition Date, Actual										200,000	400,000
Acquired intangible amortization of acquirees since acquisition										1,400,000	200,000
Business Acquisition, Pro Forma Revenue										1,716,900,000	1,585,000,000
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax										88,300,000	48,200,000
Expected forecast volatility rate [Member]
Business Acquisition [Line Items]
Business combination contingent consideration measurement input		20
FCR
Business Acquisition [Line Items]
Date of Acquisition		Oct. 26, 2019
Percentage of Voting Interests Acquired		70.00%
Contingent Consideration Arrangements, Basis for Amount		Significant assumptions include a discount rate of 16.7% expected forecast volatility of 20%, an equivalent metric risk premium of 15.1%, risk-free rate of 1.6% and a credit spread of 1.8%.
Total purchase price		$ 162,095,000
Other liabilities, noncurrent										6,100,000
Cost of Acquired Entity, Up Front Cash Consideration		107,000,000
Future Value of Liabilities Incurred From Business Acquisitions		6,500,000
Contingent Consideration, at fair value		$ 10,900,000		0						6,134,000	0
Increase (decrease) in contingent consideration payable								$ 400,000
FCR | Discount Rate
Business Acquisition [Line Items]
Business combination contingent consideration measurement input		16.7
FCR | TTEC Engage
Business Acquisition [Line Items]
Description of Acquired Entity		First Call Resolution, LLC ("FCR"), an Oregon limited liability company ("the Transaction"). FCR is a customer care, social networking and business process solutions service provider with approximately 2,000 employees based in the U.S.
FCR | Customer Relationships [Member]
Business Acquisition [Line Items]
Useful life		10 years
FCR | Trade Names [Member]
Business Acquisition [Line Items]
Useful life		4 years
SCS
Business Acquisition [Line Items]
Date of Acquisition					Apr. 30, 2018	Apr. 30, 2018
Description of Acquired Entity					Strategic Communications Services, Ltd ("SCS"). SCS provides services as a system integrator for multichannel contact center platforms, including CISCO. The Company offers in-house, managed and outsourced network, information, communications and contact center services to leading brands throughout Europe. This business has been integrated into the Company's Digital segment.	Strategic Communications Services, Ltd ("SCS"). SCS provides services as a system integrator for multichannel contact center platforms, including CISCO. The Company offers in-house, managed and outsourced network, information, communications and contact center services to leading brands throughout Europe. This business has been integrated into the Company's Digital segment.
Payments to Acquire Businesses						$ 6,100,000
Total purchase price												$ 9,511,000
Future Value of Liabilities Incurred From Business Acquisitions												2,900,000
Contingent Consideration, at fair value				2,363,000						$ 0	$ 2,363,000	4,100,000	$ 0
Contingent Consideration, at Fair Value, Current Portion												$ 2,700,000
Increase (decrease) in contingent consideration payable			$ 2,500,000	$ 300,000					$ 210,000
SCS | Discount Rate
Business Acquisition [Line Items]
Business combination contingent consideration measurement input					4.7							4.7
SCS | United Kingdom, Pounds
Business Acquisition [Line Items]
Payments to Acquire Businesses | £					£ 4.4
Contingent Consideration, at fair value | £					£ 3.0
SCS | Customer Relationships [Member]
Business Acquisition [Line Items]
Useful life					10 years	10 years
BH
Business Acquisition [Line Items]
Date of Acquisition							Mar. 31, 2018
Description of Acquired Entity							On March 31, 2018, the Company, through its subsidiary Percepta, acquired certain assets from Berkshire Hathaway Specialty Concierge, LLC ("BH") related to a customer engagement center and the related customer contracts. This acquisition is being accounted for as a business combination. These assets will be integrated into the Company's Engage segment.
Total purchase price							$ 1
BH | Customer Relationships [Member]
Business Acquisition [Line Items]
Fair Value of Assets Acquired							257,000
BH | Lease Agreements [Member]
Business Acquisition [Line Items]
Fair Value of Assets Acquired							330,000
BH | Fixed Assets
Business Acquisition [Line Items]
Fair Value of Assets Acquired							$ 98,000
Serendebyte
Business Acquisition [Line Items]
Date of Acquisition	Feb. 07, 2020
Percentage of Voting Interests Acquired	70.00%
Cost of Acquired Entity, Up Front Cash Consideration	$ 9,000,000
Serendebyte | TTEC Digital
Business Acquisition [Line Items]
Description of Acquired Entity	Serendebyte Inc., a Delaware corporation ("the Transaction"). Serendebyte is an autonomous customer experience and intelligent automation solutions provider with 125 employees based in India, the United States, and Canada.