Fair Value (Details 4) - Recurring basis - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Estimate of Fair Value Measurement
Financial Instruments Measured/Discussed at Fair Value
DownREIT units	$ (1,662)	$ (1,203)
Total	(8,296,897)	(8,312,664)
Unsecured notes | Estimate of Fair Value Measurement
Financial Instruments Measured/Discussed at Fair Value
Indebtedness	(7,280,382)	(7,271,799)
Secured Debt and Variable Rate Unsecured Indebtedness | Estimate of Fair Value Measurement
Financial Instruments Measured/Discussed at Fair Value
Indebtedness	(1,018,373)	(1,043,976)
Interest Rate Cap | Not Designated as Hedging Instrument | Estimate of Fair Value Measurement
Financial Instruments Measured/Discussed at Fair Value
Derivative assets	121	6
Interest Rate Swap | Cash Flow Hedges | Estimate of Fair Value Measurement
Financial Instruments Measured/Discussed at Fair Value
Derivative assets	3,399	4,308
Fair Value, Inputs, Level 1
Financial Instruments Measured/Discussed at Fair Value
DownREIT units	(1,662)	(1,203)
Total	(7,282,044)	(7,273,002)
Fair Value, Inputs, Level 1 | Unsecured notes
Financial Instruments Measured/Discussed at Fair Value
Indebtedness	(7,280,382)	(7,271,799)
Fair Value, Inputs, Level 2
Financial Instruments Measured/Discussed at Fair Value
Total	(1,014,853)	(1,039,662)
Fair Value, Inputs, Level 2 | Secured Debt and Variable Rate Unsecured Indebtedness
Financial Instruments Measured/Discussed at Fair Value
Indebtedness	(1,018,373)	(1,043,976)
Fair Value, Inputs, Level 2 | Interest Rate Cap | Not Designated as Hedging Instrument
Financial Instruments Measured/Discussed at Fair Value
Derivative assets	121	6
Fair Value, Inputs, Level 2 | Interest Rate Swap | Cash Flow Hedges
Financial Instruments Measured/Discussed at Fair Value
Derivative assets	3,399	4,308
Fair Value, Inputs, Level 3
Financial Instruments Measured/Discussed at Fair Value
Total	$ 0	$ 0