BUSINESS COMBINATIONS - Restructuring charges (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 3,362	$ 686	$ 4,376	$ 2,167
Costs to exit duplicative contracts [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	0	349	0	510
Workforce reduction [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	3,100	337	3,793	928
Lease abandonment costs [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring charges			0	257
Other restructuring costs [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 262	$ 0	$ 583	$ 472