SEGMENT REPORTING, ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total property and equipment, net	$ 238,866	$ 126,325
Total goodwill	2,293,514	2,235,999	$ 1,882,020
Total assets	7,131,316	6,915,420
Total liabilities	1,527,893	1,540,061
North America
Segment Reporting Information [Line Items]
Total goodwill	2,145,506	2,085,494	1,738,360
International
Segment Reporting Information [Line Items]
Total goodwill	148,008	150,505	$ 143,660
Operating Segments | North America
Segment Reporting Information [Line Items]
Total property and equipment, net	237,010	123,634
Total goodwill	2,145,506	2,085,494
Total assets	6,853,670	6,674,974
Total liabilities	1,484,686	1,496,894
Operating Segments | International
Segment Reporting Information [Line Items]
Total property and equipment, net	1,856	2,691
Total goodwill	148,008	150,505
Total assets	277,646	240,446
Total liabilities	$ 43,207	$ 43,167