PROPERTY AND EQUIPMENT - Summary of Depreciation and Amortization of Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended											6 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Costs of sales and services																		$ 76,199	$ 73,738	$ 68,804
Other depreciation and amortization																		5,013	10,644	2,478
Subtotal																		81,212	84,382	71,282
Amortization of acquired and developed technologies																		4,501	4,034	2,147
Total depreciation and amortization	$ 20,999	$ 21,505	$ 22,176	$ 22,710	$ 23,009	$ 22,179	$ 20,523	$ 16,793	$ 20,320	$ 18,755	$ 17,561	$ 43,321	$ 42,702	$ 36,316	$ 64,320	$ 65,710	$ 56,636	$ 85,713	$ 88,416	$ 73,429