INCOME TAXES - Effective tax rate reconciliation (Details)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
INCOME TAXES
U.S. Federal statutory tax rate (as a percent)	21.00%	21.00%	21.00%	35.00%
Income passed through to individual members (as a percent)		(11.30%)	(31.90%)	(36.80%)
Foreign income taxes (as a percent)		(31.00%)	(6.20%)	(1.00%)
Non-cash equity compensation expense (as a percent)		(41.80%)	(5.80%)
Impairment of equity method investment		(31.00%)
Other non-deductible expenses		(19.20%)	(1.60%)	(0.30%)
Valuation allowance (as a percent)			0.10%	1.60%
State and local income taxes, net of U.S federal tax benefit (as a percent)		(32.60%)	(6.10%)	(0.70%)
Remeasurement of deferred taxes (as a percent)				5.30%
Other (as a percent)		4.20%	0.60%	(0.10%)
Effective income tax rate (as a percent)		(141.70%)	(29.90%)	3.00%