Fair Value (Details 2) (Recurring basis, USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Total
Financial Instruments Measured at Fair Value
DownREIT units	$ (1,020)
Indebtedness	(4,064,394)
Total	(4,125,946)
Total | Puts
Financial Instruments Measured at Fair Value
Fair value of remaining outstanding Put	(5,748)
Total | Interest Rate Caps
Financial Instruments Measured at Fair Value
Derivative assets	28
Total | Interest Rate Swaps
Financial Instruments Measured at Fair Value
Derivative liabilities	(54,812)
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Instruments Measured at Fair Value
DownREIT units	(1,020)
Indebtedness	(2,070,262)
Total	(2,071,282)
Significant Other Observable Inputs (Level 2)
Financial Instruments Measured at Fair Value
Indebtedness	(1,994,132)
Total	(2,048,916)
Significant Other Observable Inputs (Level 2) | Interest Rate Caps
Financial Instruments Measured at Fair Value
Derivative assets	28
Significant Other Observable Inputs (Level 2) | Interest Rate Swaps
Financial Instruments Measured at Fair Value
Derivative liabilities	(54,812)
Significant Unobservable Inputs (Level 3)
Financial Instruments Measured at Fair Value
Total	(5,748)
Significant Unobservable Inputs (Level 3) | Puts
Financial Instruments Measured at Fair Value
Fair value of remaining outstanding Put	$ (5,748)