FAIR VALUE MEASUREMENTS AND THE FAIR VALUE OPTION - Schedule of the Funds and Fair Value Option Investments, Significant Inputs (Details)	Dec. 31, 2018
Measurement Input, Cap Rate [Member] | Level 3 [Member] | Multifamily [Member] | Minimum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0450
Measurement Input, Cap Rate [Member] | Level 3 [Member] | Multifamily [Member] | Maximum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0775
Measurement Input, Cap Rate [Member] | Level 3 [Member] | Office [Member] | Minimum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0475
Measurement Input, Cap Rate [Member] | Level 3 [Member] | Office [Member] | Maximum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0650
Measurement Input, Cap Rate [Member] | Level 3 [Member] | Retail [Member] | Minimum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0525
Measurement Input, Cap Rate [Member] | Level 3 [Member] | Retail [Member] | Maximum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.1000
Measurement Input, Cap Rate [Member] | Level 3 [Member] | Hotel [Member] | Minimum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0600
Measurement Input, Cap Rate [Member] | Level 3 [Member] | Hotel [Member] | Maximum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0775
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Multifamily [Member] | Minimum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0775
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Multifamily [Member] | Maximum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0975
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Office [Member] | Minimum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0700
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Office [Member] | Maximum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0800
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Retail [Member] | Minimum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0650
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Retail [Member] | Maximum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.1175
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Hotel [Member] | Minimum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0825
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Hotel [Member] | Maximum [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.1000
Measurement Input, Discount Rate [Member] | Level 3 [Member] | Residential [Member] | Income and Market Approach Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.1200
Measurement Input, Credit Spread [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0133
Measurement Input, Credit Spread [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0346