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FAIR VALUE MEASUREMENTS - Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 646,432	$ 432,954	$ 441,561	$ 440,430
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	646,432	432,954
Unsecured credit facility	250,000
Carrying Amount \| Mortgages
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgages payable	1,600,590	1,556,248
Unamortized debt issuance costs	(10,300)	(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	646,432	432,954
Fair Value \| Level 2 \| Mortgages
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgages payable	$ 1,617,137	$ 1,590,503