EQUITY STRUCTURE AND ACCOUNTS - Dividends Declared (Details) - $ / shares															3 Months Ended								12 Months Ended
	Jan. 24, 2019	Nov. 01, 2018	Sep. 05, 2018	May 30, 2018	Feb. 27, 2018	Jan. 03, 2018	Nov. 07, 2017	Sep. 01, 2017	Jun. 01, 2017	Mar. 01, 2017	Dec. 02, 2016	Sep. 01, 2016	Jun. 01, 2016	Mar. 01, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Dividends per share of Class A common stock (in dollars per share)						$ 0.315													$ 0.315	$ 0.3	$ 0.3	$ 0.3	$ 1.535	$ 1.215	$ 1.285
Class A Common Stock
Class of Stock [Line Items]
Dividends per share of Class A common stock (in dollars per share)		$ 0.57	$ 0.325	$ 0.325	$ 0.315		$ 0.315	$ 0.3	$ 0.3	$ 0.3	$ 0.460	$ 0.275	$ 0.275	$ 0.275	$ 0.57	$ 0.325	$ 0.325	$ 0.315					$ 1.535	$ 1.215	$ 1.285
Subsequent Event | Class A Common Stock
Class of Stock [Line Items]
Dividends per share of Class A common stock (in dollars per share)	$ 0.570