SEGMENT AND GEOGRAPHICAL REPORTING - Revenues and Property and Equipment by Geographical Location (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenue	$ 311,000	$ 308,000	$ 311,200	$ 285,900	$ 265,700	$ 251,900	$ 250,700	$ 238,500	$ 222,300	$ 230,000	$ 256,200	$ 253,600	$ 1,216,197	$ 1,006,782	$ 962,092
Property and equipment, net	51,330				53,337								51,330	53,337
Reportable Geographical Components | United States
Segment Reporting Information [Line Items]
Total revenue													1,110,000	903,000	874,000
Property and equipment, net	41,000				43,000								41,000	43,000
Reportable Geographical Components | Canada
Segment Reporting Information [Line Items]
Total revenue													106,000	104,000	88,000
Property and equipment, net	10,000				10,000								10,000	10,000
Reportable Geographical Components | Other
Segment Reporting Information [Line Items]
Total revenue													0	0	$ 0
Property and equipment, net	$ 0				$ 0								$ 0	$ 0