ACQUISITIONS (Fair Values of Net Assets Acquired and Liabilities Assumed) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Apr. 30, 2019	Dec. 31, 2018	Nov. 01, 2018	Mar. 15, 2018
Business Acquisition [Line Items]
Goodwill	$ 211,956	$ 195,043		$ 166,832
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 9)						3,220
Other noncurrent liabilities						490
Total liabilities assumed						7,456
Net assets acquired						$ 54,839
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 9)					0
Other noncurrent liabilities					0
Total liabilities assumed					2,025
Net assets acquired					$ 32,429
test IO
Business Acquisition [Line Items]
Cash and cash equivalents			$ 663
Trade receivables and contract assets			621
Prepaid and other current assets			150
Goodwill			11,926
Intangible assets			6,219
Property and equipment and other noncurrent assets			305
Total assets acquired			19,884
Accounts payable, accrued expenses and other current liabilities			993
Long-term debt (Note 9)			0
Other noncurrent liabilities			1,568
Total liabilities assumed			2,561
Net assets acquired			$ 17,323
Other 2019 Acquisitions
Business Acquisition [Line Items]
Intangible assets		$ 7,500
2020 Acquisitions
Business Acquisition [Line Items]
Intangible assets	$ 7,300