SIGNIFICANT ACCOUNTING POLICIES - Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New accounting pronouncements
(Increase)/decrease in operating assets	$ (13,880)	$ (9,008)	$ (29,399)
Net cash provided by/(used in) operating activities	560,676	518,915	536,568
Proceeds from sales of real estate investments, net	247,031	71,235	301,799
Capital expenditures and other major improvements - real estate assets, net of escrow reimbursement	112,359	124,728	91,740
Net cash provided by/(used in) investing activities	(113,548)	(407,406)	(112,720)
Net increase/(decrease) in cash, cash equivalents, and restricted cash	$ 187,061	(276)	(5,434)
ASU 2016-18 | Adjustment
New accounting pronouncements
(Increase)/decrease in operating assets		(237)	(361)
Net cash provided by/(used in) operating activities		(237)	(361)
Proceeds from sales of real estate investments, net			(555)
Capital expenditures and other major improvements - real estate assets, net of escrow reimbursement		35	112
Net cash provided by/(used in) investing activities		35	(443)
Net increase/(decrease) in cash, cash equivalents, and restricted cash		$ (202)	$ (804)