FAIR VALUE DISCLOSURES - Face Values, Carrying Amounts and Fair Values of Financial Instruments (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Loans receivable	$ 89,767,000	$ 91,280,000
Preferred equity investments	46,616,000	48,035,000
Senior Notes
Financial liabilities:
Financial liabilities	1,300,000,000	1,300,000,000
Secured Indebtedness
Financial liabilities:
Financial liabilities	255,512,000	259,202,000
Carrying Amount
Financial assets:
Loans receivable	63,255,000	65,907,000
Preferred equity investments	47,096,000	48,483,000
Carrying Amount | Senior Notes
Financial liabilities:
Financial liabilities	1,307,120,000	1,306,286,000
Carrying Amount | Secured Indebtedness
Financial liabilities:
Financial liabilities	252,827,000	256,430,000
Fair Value
Financial assets:
Loans receivable	66,440,000	65,892,000
Preferred equity investments	46,575,000	47,064,000
Fair Value | Senior Notes
Financial liabilities:
Financial liabilities	1,300,363,000	1,329,191,000
Fair Value | Secured Indebtedness
Financial liabilities:
Financial liabilities	$ 234,999,000	$ 246,461,000