EQUITY STRUCTURE AND ACCOUNTS - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2018		Mar. 31, 2017	Dec. 31, 2017
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	$ 1,488,146		$ 1,509,554	$ 1,509,554
Other comprehensive income (loss)	(8,857)		4,751	(2,220)
Exchange of noncontrolling interest for common stock	313			(2,353)
Rebalancing of ownership percentage between Company and Operating Partnership	(95)		4,219
Ending Balance	1,503,292	[1]		1,488,146
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(212)		1,365	1,365
Other comprehensive income (loss)	(7,528)		3,117	(2,915)
Exchange of noncontrolling interest for common stock	(143)		403	1,696
Rebalancing of ownership percentage between Company and Operating Partnership	3		(51)
Ending Balance	(7,880)		4,834	(212)
Accumulated Other Comprehensive Income of Noncontrolling Interests
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	116		761	761
Other comprehensive income (loss)	(1,329)		1,634
Exchange of noncontrolling interest for common stock	143		(403)
Rebalancing of ownership percentage between Company and Operating Partnership	(3)		51
Ending Balance	(1,073)		2,043	116
Total Accumulated Other Comprehensive Income
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(96)		2,126	2,126
Other comprehensive income (loss)	(8,857)		4,751
Exchange of noncontrolling interest for common stock	0		0
Rebalancing of ownership percentage between Company and Operating Partnership	0		0
Ending Balance	$ (8,953)		$ 6,877	$ (96)

[1] Includes amounts relating to consolidated variable interest entities. See Note 3.