EQUITY STRUCTURE AND ACCOUNTS - Schedule of Treasury Stock Repurchase Activity (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Oct. 30, 2014
Shares Repurchase Program [Roll Forward]
Repurchases paid			$ (4,652,000)
2014 Share Repurchase Authorization Program | Class A common stock
Shares Repurchase Program [Roll Forward]
Authorizations remaining at beginning of the year	$ 44,353,000	$ 49,006,000	49,006,000
Additional authorizations	$ 0	$ 0		$ 50,000,000.0
Repurchases paid (in shares)	0	424,317
Repurchases paid	$ 0	$ (4,653,000)
Repurchases unsettled	0	0
Authorizations remaining at the end of the year	$ 44,353,000	$ 44,353,000	$ 44,353,000