ACQUISITIONS AND DIVESTITURES (NARRATIVE) (DETAILS) $ in Thousands, $ in Millions				3 Months Ended									12 Months Ended
	Nov. 08, 2017 USD ($)	Apr. 03, 2017 USD ($)	Nov. 09, 2016 CAD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 09, 2016 USD ($)
Business Acquisition [Line Items]
Other liabilities, noncurrent					$ 95,243			$ 71,664					$ 95,243	$ 71,664
Contingent Consideration, at fair value					399			1,808		$ 13,450			399	1,808	$ 13,450
Revenue of Acquirees since Acquisition Date, Actual													172,300	10,000
Income (loss) from operations of Acquirees since Acquisition Date, Actual													(700)	(300)
Proceeds from sale of business													636	0	0
Gain on sale of business													908	0	0
Other nonoperating income expense													1,869	9,555	2,157
Business Combination Pro Forma Information Amortization Expense of Acquirees Since Acquisition													3,500	100
Business Acquisition, Pro Forma Revenue													1,541,600	1,489,100
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax													11,500	31,100
Investments in non-marketable equity investments													1,384	3,179	9,000
Motif [Member]
Business Acquisition [Line Items]
Percentage of Voting Interests Acquired	70.00%
Description of Acquired Entity	Motif, Inc., a California corporation ("Motif"). Motif is a digital trust and safety services company serving eCommerce marketplaces, online retailers, travel agencies and financial services companies. Motif provides omni-channel community moderation services via voice, email and chat from delivery centers in India and the Philippines via approximately 2,700 employees
Payments to Acquire Businesses	$ 46,800
Total purchase price	$ 73,438
Other liabilities, noncurrent					27,800								$ 27,800
Contingent Consideration Arrangements, Basis for Amount	The Company also agreed to purchase the remaining 30% interest in Motif from Motif's founders ("founders' shares") no later than May 2020 ("30% buyout period"). The Company agreed to pay for the founders' shares at a purchase price based on Motif's fiscal year 2020's adjusted normalized EBITDA, $5.0 million in cash, and 30% of the excess cash present in the business at the time of the buyout; or if the buyout occurs prior to May 2020, the trailing twelve months EBITDA, calculated from the most recently completed full monthly period ending prior to the date of the buyout triggering event, $5.0 million in cash, and 30% of the excess cash in the business at that point.
Motif [Member] | Customer Relationships
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	11 years
Connextions
Business Acquisition [Line Items]
Description of Acquired Entity		Connextions, Inc., a health care customer service provider company, from OptumHealth Holdings, LLC. Connextions is being integrated into the health care vertical of the CMS segment of the Company. Connextions employed approximately 2,000 at several centers in the U.S.
Payments to Acquire Businesses		$ 80,000
Total purchase price		$ 77,712
Payments for Previous Acquisition						$ 1,800
Proceeds from Previous Acquisition						$ 4,100
Connextions | Customer Relationships
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		12 years
Atelka [Member]
Business Acquisition [Line Items]
Date of Acquisition			Nov. 09, 2016
Description of Acquired Entity			On November 9, 2016, the Company acquired all of the outstanding shares of Atelka Enterprise Inc. ("Atelka"), a Canadian customer contact center management and business process outsourcing services company that serves Canadian telecommunications, logistics, and entertainment clients. This acquisition was an addition to the CMS segment. Atelka employs approximately 2,800 in Quebec, Ontario, New Brunswick and Prince Edward Island.
Total purchase price																$ 48,401
Cost of Acquired Entity, Up Front Cash Consideration																47,500
Acquisition hold-back payment																$ 1,400
Discount rate													0.00%
Contingent Consideration, at fair value					0			558		0			$ 0	558	0
Increase (decrease) in contingent consideration payable					$ 600
Atelka [Member] | CAD
Business Acquisition [Line Items]
Total purchase price			$ 65.0
Increase (decrease) in contingent consideration payable				$ 0.8
Atelka [Member] | Customer Relationships
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			12 years
rogenSi
Business Acquisition [Line Items]
Discount rate													4.60%
Contingent Consideration, at fair value								0		9,797				0	9,797
Increase (decrease) in contingent consideration payable									$ 4,300	300	$ 800
Sofica [Member]
Business Acquisition [Line Items]
Discount rate													5.00%
Contingent Consideration, at fair value								$ 0		$ 3,153				$ 0	$ 3,153
Increase (decrease) in contingent consideration payable												$ 500
CafeX [Member]
Business Acquisition [Line Items]
Equity Method Investment, Ownership Percentage					17.20%			17.20%					17.20%	17.20%
Investments in non-marketable equity investments							$ 1,100	$ 4,300