EQUITY STRUCTURE AND ACCOUNTS - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	$ 1,509,554		$ 1,491,408	$ 1,505,207
Other comprehensive income (loss)	(2,220)		8,519	(36,545)
Exchange of noncontrolling interest for common stock	(2,353)		0	0
Rebalancing of ownership percentage between Company and Operating Partnership	(3,524)		8,096	(544)
Ending Balance	1,488,146	[1]	1,509,554	1,491,408
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	1,365		(3,556)	15,656
Other comprehensive income (loss)	(2,915)		3,420	(20,046)
Exchange of noncontrolling interest for common stock	1,696		1,202	645
Rebalancing of ownership percentage between Company and Operating Partnership	(358)		299	189
Ending Balance	(212)		1,365	(3,556)
Accumulated Other Comprehensive Income of Noncontrolling Interests
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	759		(2,839)	14,494
Other comprehensive income (loss)	695		5,099	(16,499)
Exchange of noncontrolling interest for common stock	(1,696)		(1,202)	(645)
Rebalancing of ownership percentage between Company and Operating Partnership	358		(299)	(189)
Ending Balance	116		759	(2,839)
Total Accumulated Other Comprehensive Income
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	2,124		(6,395)	30,150
Other comprehensive income (loss)	(2,220)		8,519	(36,545)
Exchange of noncontrolling interest for common stock	0		0	0
Rebalancing of ownership percentage between Company and Operating Partnership	0		0	0
Ending Balance	$ (96)		$ 2,124	$ (6,395)

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