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FAIR VALUE MEASUREMENTS - Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 615,579	$ 432,954	$ 412,126	$ 440,430
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	615,579	432,954
Unsecured credit facility	250,000
Carrying Amount \| Mortgages
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgages payable	1,595,235	1,556,248
Unamortized debt issuance costs	(10,500)	(10,100)
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unsecured credit facility	250,000
Fair Value \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	615,579	432,954
Fair Value \| Level 2 \| Mortgages
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgages payable	$ 1,628,764	$ 1,590,503