Fair Value Measurements - (Details 1) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of real estate (held for use or held for sale) prior to recognition of impairment losses	$ 36,900,000		$ 81,800,000
Fair value measurement, impairment losses on real estate	35,100,000		65,800,000
Provision for real estate impairment losses	1,820,000	0	6,235,000
Impairment losses on real estate			16,000,000
Unlevered FCF rate of return of properties impaired	5.08%		7.39%
Estimated fair value of consolidated debt	4,400,000,000	4,500,000,000
Total indebtedness	4,135,139,000	4,388,185,000
Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Swap Notional Amount	50,300,000	50,700,000
Weighted Average Remaining Term of interest Rate Swaps	6 years 0 months
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	1,700,000
Weighted average fixed receive rate under total rate of return swaps	3.43%
Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Face Value	100,900,000
Excepted Remaining Term Available For Sale Securities	6 years 5 months
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost of the investment in securities	$ 63,600,000	$ 59,800,000
Impaired asset [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of apartment communities	1		9