REAL ESTATE AND RELATED LEASE INTANGIBLES, NET - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Gross intangible assets	$ 189,500	$ 154,700
Accumulated amortization	60,900	48,100
Net intangible assets	128,600	106,600
Unamortized favorable/unfavorable lease intangibles	8,900	7,000
Net increase in operating lease income, amortization of below market lease intangibles	1,900	1,400	$ 1,800
Unbilled rent receivables	900	700
Unencumbered real estates	$ 128,700	$ 70,300
Weighted average amortization period for intangible assets acquired	19 years	19 years 6 months	22 years 7 months 6 days
Revenue recorded from acquisitions	$ 9,500	$ 2,800	$ 14,000
Earnings (losses) recorded from acquisitions	6,800	(300)	3,200
Real estate acquired in settlement of mortgage loan receivable held for investment	0	0	6,700
Mortgage loans on real estate, carrying amount	3,512,642
Above Market Leases
Business Acquisition [Line Items]
Net reduction in operating lease income, amortization of above market leases	$ 1,100	$ 1,300	1,400
Other
Business Acquisition [Line Items]
Real estate acquired in settlement of mortgage loan receivable held for investment			6,700
Gain (loss) on sale of mortgage loans			800
Mortgage loans on real estate, carrying amount			$ 5,900