CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jan. 31, 2016	Dec. 31, 2015
Supplemental disclosures of non-cash investing and financing activities
Common stock issued through the dividend reinvestment plan (in shares)			1,689	1,608
Common stock issued through the dividend reinvestment plan			$ 304	$ 275
Number of shares withheld to satisfy employees' tax withholding and other liabilities (in shares)			53,214	39,800
Shares withheld to satisfy employees' tax withholding and other liabilities, value			$ 8,316	$ 5,921
Stock Issued During Period, Shares, Share-based Compensation, Forfeited (in shares)			407	4,293
Stock Issued During Period, Value, Share-based Compensation, Forfeited			$ 627	$ 502
Dividends declared but not paid	$ 185,384	$ 171,098	185,384	171,098		$ 171,257
Change in redemption value of redeemable noncontrolling interest			(529)
Increase (Decrease) in Prepaid Expense and Other Assets			(2,689)
Income (loss) on cash flow hedges	719	(31)	(73,826)	(67)
Increase in accrued expenses, other liabilities and accrued interest payable			53,591
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax			(56,280)
Cash flow hedge losses reclassified to earnings	1,748	1,373	4,682	4,401
Impairment of Real Estate	0	$ 658	(3,935)	(10,668)
New England [Member]
Supplemental disclosures of non-cash investing and financing activities
Impairment of Real Estate				4,195
Avalon at Edgewater [Member]
Supplemental disclosures of non-cash investing and financing activities
Impairment of Real Estate			(8,702)	(15,663)
Avalon at Edgewater [Member] | New England [Member]
Supplemental disclosures of non-cash investing and financing activities
Impairment of Real Estate				$ 26,039
Mortgage notes payable | Notes Payable Maturities 2020 [Member] | Avalon Hoboken [Member]
Supplemental disclosures of non-cash investing and financing activities
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt					$ 67,904
Mortgage notes payable | Notes Payable Maturities 2019 [Member] | Avalon Columbia Pike [Member]
Supplemental disclosures of non-cash investing and financing activities
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	$ 70,507		$ 70,507
Restricted Stock Converted From Performance Shares [Member]
Supplemental disclosures of non-cash investing and financing activities
Restricted stock granted (in shares)			115,618	95,826
Restricted stock
Supplemental disclosures of non-cash investing and financing activities
Restricted stock granted (in shares)			80,873	61,953
Fair value of shares issued			$ 13,129	$ 10,721
Stock Issued During Period, Shares, Share-based Compensation, Forfeited (in shares)			3,848
Restricted Stock and Restricted Stock Converted From Performance Shares [Member]
Supplemental disclosures of non-cash investing and financing activities
Restricted stock granted (in shares)			196,491	157,779
Retained Earnings [Member]
Supplemental disclosures of non-cash investing and financing activities
Change in redemption value of redeemable noncontrolling interest			$ (529)	$ 1,722
Deferred Compensation, Share-based Payments [Member] | Non Employee Director [Member]
Supplemental disclosures of non-cash investing and financing activities
Fair value of shares issued			$ 3,894	$ 3,552
Non Employee Director [Member] | Deferred Compensation, Share-based Payments [Member]
Supplemental disclosures of non-cash investing and financing activities
Restricted stock granted (in shares)			44,327	46,589