FAIR VALUE OF DERIVATIVES AND FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Fair Value
Notes receivable, net	$ 156,996	$ 153,650
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Gross Amounts of Recognized Assets	2	6
Gross Amounts of Recognized Liabilities	1,236	142
Unsecured debt instruments
Redeemable Noncontrolling Interest, Equity, Carrying Amount	759,986	1,018,665
Redeemable noncontrolling interests in the Operating Partnership and DownREIT Partnership	759,986	1,018,665
Transfer Between the Levels	0
Carrying Amount \| Fair Value, Measurements, Recurring
Fair Value
Notes receivable, net	156,996	153,650
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	156,998	153,656
Unsecured debt instruments
Total liabilities	4,945,692	4,731,595
Redeemable noncontrolling interests in the Operating Partnership and DownREIT Partnership	759,986	1,018,665
Carrying Amount \| Interest rate contracts \| Fair Value, Measurements, Recurring
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivative Asset Designated as Hedging Instrument, Fair Value	2	6
Derivatives - Interest rate contracts (b)	1,236	142
Carrying Amount \| Credit facilities \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments	22,086
Carrying Amount \| Commercial bank \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments		16,583
Commercial paper program	230,000	300,000
Carrying Amount \| Senior Unsecured Notes \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments	3,756,877	3,263,152
Fair Value \| Fair Value, Measurements, Recurring
Fair Value
Notes receivable, net	168,122	160,197
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	168,124	160,203
Unsecured debt instruments
Total liabilities	5,333,919	4,853,337
Redeemable noncontrolling interests in the Operating Partnership and DownREIT Partnership	759,986	1,018,665
Fair Value \| Interest rate contracts \| Fair Value, Measurements, Recurring
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivative Asset Designated as Hedging Instrument, Fair Value	2	6
Derivatives - Interest rate contracts (b)	1,236	142
Fair Value \| Credit facilities \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments	22,086
Fair Value \| Commercial bank \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments		16,583
Commercial paper program	230,000	300,000
Fair Value \| Senior Unsecured Notes \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments	4,094,384	3,397,622
Fair Value \| Level 2 \| Fair Value, Measurements, Recurring
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	2	6
Unsecured debt instruments
Total liabilities	1,236	142
Redeemable noncontrolling interests in the Operating Partnership and DownREIT Partnership	759,986	1,018,665
Fair Value \| Level 2 \| Interest rate contracts \| Fair Value, Measurements, Recurring
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivative Asset Designated as Hedging Instrument, Fair Value	2	6
Derivatives - Interest rate contracts (b)	1,236	142
Fair Value \| Level 3 \| Fair Value, Measurements, Recurring
Fair Value
Notes receivable, net	168,122	160,197
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	168,122	160,197
Unsecured debt instruments
Total liabilities	5,332,683	4,853,195
Fair Value \| Level 3 \| Credit facilities \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments	22,086
Fair Value \| Level 3 \| Commercial bank \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments		16,583
Commercial paper program	230,000	300,000
Fair Value \| Level 3 \| Senior Unsecured Notes \| Fair Value, Measurements, Recurring
Unsecured debt instruments
Unsecured debt instruments	4,094,384	3,397,622
Tax-exempt secured notes payable \| Variable Rate Debt
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Secured debt including debt on real estate held for sale	27,000	27,000
Tax-exempt secured notes payable \| Variable Rate Debt \| Carrying Amount \| Tax-exempt secured notes payable \| Fair Value, Measurements, Recurring
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Secured debt including debt on real estate held for sale	27,000	27,000
Tax-exempt secured notes payable \| Variable Rate Debt \| Fair Value \| Tax-exempt secured notes payable \| Fair Value, Measurements, Recurring
Debt instruments - fair value
Fair value	27,000	27,000
Tax-exempt secured notes payable \| Variable Rate Debt \| Fair Value \| Level 3 \| Tax-exempt secured notes payable \| Fair Value, Measurements, Recurring
Debt instruments - fair value
Fair value	27,000	27,000
Credit facilities \| Fixed Rate Debt
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Secured debt including debt on real estate held for sale		204,590
Credit facilities \| Fixed Rate Debt \| Carrying Amount \| Credit facilities \| Fair Value, Measurements, Recurring
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Secured debt including debt on real estate held for sale		218,490
Credit facilities \| Fixed Rate Debt \| Fair Value \| Credit facilities \| Fair Value, Measurements, Recurring
Debt instruments - fair value
Fair value		213,661
Credit facilities \| Fixed Rate Debt \| Fair Value \| Level 3 \| Credit facilities \| Fair Value, Measurements, Recurring
Debt instruments - fair value
Fair value		213,661
Mortgages Note Payable \| Fixed Rate Debt
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Secured debt including debt on real estate held for sale	892,291	884,869
Mortgages Note Payable \| Fixed Rate Debt \| Carrying Amount \| Mortgages Note Payable \| Fair Value, Measurements, Recurring
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Secured debt including debt on real estate held for sale	908,493	906,228
Mortgages Note Payable \| Fixed Rate Debt \| Fair Value \| Mortgages Note Payable \| Fair Value, Measurements, Recurring
Debt instruments - fair value
Fair value	959,213	898,329
Mortgages Note Payable \| Fixed Rate Debt \| Fair Value \| Level 3 \| Mortgages Note Payable \| Fair Value, Measurements, Recurring
Debt instruments - fair value
Fair value	$ 959,213	$ 898,329

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