Fair Value (Details 2) (Recurring basis, USD $) In Thousands, unless otherwise specified	Mar. 31, 2015
Fair Value, Inputs, Level 1 [Member]
Financial Instruments Measured/Discussed at Fair Value
DownREIT units	$ (1,307)
Total	(2,898,853)
Fair Value, Inputs, Level 2 [Member]
Financial Instruments Measured/Discussed at Fair Value
Total	(3,718,759)
Fair Value, Inputs, Level 3 [Member]
Financial Instruments Measured/Discussed at Fair Value
Total	(8,765)
Fair Value, Inputs, Level 3 [Member] | Puts
Financial Instruments Measured/Discussed at Fair Value
Fair value of remaining outstanding Puts	(8,765)
Cash Flow Hedges | Fair Value, Inputs, Level 2 [Member] | Interest Rate Cap [Member]
Financial Instruments Measured/Discussed at Fair Value
Derivative assets	36
Unsecured notes | Fair Value, Inputs, Level 1 [Member]
Financial Instruments Measured/Discussed at Fair Value
Indebtedness	(2,897,546)
Secured Debt and Variable Rate Unsecured Term Loan [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments Measured/Discussed at Fair Value
Indebtedness	(3,718,836)
Not Designated as Hedging Instrument [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate Cap [Member]
Financial Instruments Measured/Discussed at Fair Value
Derivative assets	41
Estimate of Fair Value Measurement [Member]
Financial Instruments Measured/Discussed at Fair Value
DownREIT units	(1,307)
Total	(6,626,377)
Estimate of Fair Value Measurement [Member] | Puts
Financial Instruments Measured/Discussed at Fair Value
Fair value of remaining outstanding Puts	(8,765)
Estimate of Fair Value Measurement [Member] | Cash Flow Hedges | Interest Rate Cap [Member]
Financial Instruments Measured/Discussed at Fair Value
Derivative assets	36
Estimate of Fair Value Measurement [Member] | Unsecured notes
Financial Instruments Measured/Discussed at Fair Value
Indebtedness	2,897,546
Estimate of Fair Value Measurement [Member] | Secured Debt and Variable Rate Unsecured Term Loan [Member]
Financial Instruments Measured/Discussed at Fair Value
Indebtedness	3,718,836
Estimate of Fair Value Measurement [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Cap [Member]
Financial Instruments Measured/Discussed at Fair Value
Derivative assets	$ 41