STOCKHOLDERS EQUITY (Details) $ / shares in Units, $ in Thousands	1 Months Ended				12 Months Ended			33 Months Ended
	Jul. 31, 2017 $ / shares shares	Jan. 31, 2017 $ / shares shares	Nov. 30, 2014 $ / shares shares	Apr. 30, 2014 shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Jul. 31, 2017 shares
Stockholders Equity
Treasury stock shares acquired (in shares)					319,526	124,268
Treasury stock shares acquired | $					$ 12,258	$ 3,314	$ 7,616
Group LP
Stockholders Equity
Number of units held by noncontrolling interest holders					22,472,337	33,698,300
Noncontrolling interests (as a percent)					40.00%	62.00%
Class A common stock
Stockholders Equity
Total Increase in Shares Outstanding				15,263,653
Common stock, shares issued					34,163,042	20,948,998
Common stock, shares outstanding					33,455,626	20,561,108
Number of shares of common stock to be issued upon exchange of a partnership unit				1	1
Class A common stock | Follow-on Offering
Stockholders Equity
Total Shares Offered	6,000,000	5,750,000	6,325,000					18,075,000
Total Increase in Shares Outstanding	5,680,903	5,356,876	4,511,058					15,548,837
Class A common stock | Group LP
Stockholders Equity
Common stock, shares outstanding					33,455,626
Class B common stock
Stockholders Equity
Total Increase in Shares Outstanding				36,158,698
Common stock, shares issued					19,912,230	31,138,193
Common stock, shares outstanding					19,912,230	31,138,193
Ratio of subscription price to the initial public offering price of shares of common stock				0.00055
Dividends payable ratio to outstanding shares of publicly traded common stock				0.00055
Class B common stock | Follow-on Offering
Stockholders Equity
Shares converted			2,998,322
Shares issued upon conversion			1,658
Conversion ratio			0.00055
Number of shares repurchased and retired	5,680,903	5,356,876	1,509,131
Price per share of stock which was repurchased and retired | $ / shares	$ 128	$ 101	$ 28