EQUITY STRUCTURE AND ACCOUNTS - Schedule of repurchase of treasury stock activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 30, 2014
Treasury Stock [Roll Forward]
Repurchases paid		$ 2,588,000	$ 4,652,000
2014 Share Repurchase Authorization Program
Treasury Stock [Roll Forward]
Remaining amount available for repurchase	$ 41,800,000
2014 Share Repurchase Authorization Program | Class A Common Stock
Class of Stock [Line Items]
Purchase of treasury stock (in shares)	0	189,897	424,317
Treasury Stock [Roll Forward]
Remaining amount available for repurchase	$ 41,769,000	$ 44,353,000	$ 49,006,000
Additional authorizations	0	0	0	$ 50,000,000
Repurchases paid	0	2,584,000	4,653,000
Repurchases unsettled	0	0	0
Remaining amount available for repurchase	$ 41,769,000	$ 41,769,000	$ 44,353,000