ACQUISITIONS (Fair Values of Net Assets Acquired and Liabilities Assumed) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Apr. 30, 2019	Nov. 01, 2018	Mar. 15, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 195,043	$ 166,832				$ 119,531
Continuum
Business Acquisition [Line Items]
Cash and cash equivalents					$ 2,251
Trade receivables and contract assets					9,139
Prepaid and other current assets					936
Goodwill					26,617
Intangible assets					14,450
Property and equipment and other noncurrent assets					8,902
Total assets acquired					62,295
Accounts payable, accrued expenses and other current liabilities					3,746
Long-term debt (Note 8)					3,220
Other noncurrent liabilities					490
Total liabilities assumed					7,456
Net assets acquired					$ 54,839
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Purchase price adjustment		(76)
Total net assets purchase accounting adjustment		$ (76)
Think
Business Acquisition [Line Items]
Cash and cash equivalents				$ 2,344
Trade receivables and contract assets				2,637
Prepaid and other current assets				900
Goodwill				20,477
Intangible assets				6,882
Property and equipment and other noncurrent assets				1,214
Total assets acquired				34,454
Accounts payable, accrued expenses and other current liabilities				2,025
Long-term debt (Note 8)				0
Other noncurrent liabilities				0
Total liabilities assumed				2,025
Net assets acquired				$ 32,429
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Purchase price adjustment	185
Total net assets purchase accounting adjustment	185
Increase in deferred tax assets	1,497
Goodwill purchase accounting adjustment	(2,043)
test IO
Business Acquisition [Line Items]
Cash and cash equivalents			$ 663
Trade receivables and contract assets			688
Prepaid and other current assets			96
Goodwill			12,150
Intangible assets			6,219
Property and equipment and other noncurrent assets			151
Total assets acquired			19,967
Accounts payable, accrued expenses and other current liabilities			910
Long-term debt (Note 8)			0
Other noncurrent liabilities			1,734
Total liabilities assumed			2,644
Net assets acquired			$ 17,323
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Purchase price adjustment	119
Total net assets purchase accounting adjustment	119
Intangible assets purchase accounting adjustment	(145)
Goodwill purchase accounting adjustment	145
Other 2019 Acquisitions
Business Acquisition [Line Items]
Intangible assets	$ 7,488