STOCKHOLDERS' AND MEMBERS' EQUITY STRUCTURE Treasury Stock (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Treasury stock, at cost, 6,050,717 shares outstanding at December 31, 2015 and 5,765,946 shares outstanding at December 31, 2014		$ 152,585	$ 151,547
Treasury Stock, Value, Acquired, Cost Method		$ 1,038
Class A Common Stock [Member]
Class of Stock [Line Items]
Treasury stock, at cost, 6,132,884 shares outstanding at December 31, 2017 and 6,087,963 shares outstanding at December 31, 2016		6,132,884	6,087,963
Repurchase Program [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Treasury stock, at cost, 6,132,884 shares outstanding at December 31, 2017 and 6,087,963 shares outstanding at December 31, 2016	[1]	6,050,717
Treasury stock, at cost, 6,050,717 shares outstanding at December 31, 2015 and 5,765,946 shares outstanding at December 31, 2014		$ 150,804

[1] The shares purchased include only those shares that have settled as of the period end date.