Organization, Basis of Presentation and Significant Accounting Policies (Details 2) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) community home building $ / shares shares	Sep. 30, 2014 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) community Claim home Citation plaintiff building $ / shares shares	Sep. 30, 2014 USD ($) $ / shares shares
Loss Contingencies [Line Items]
Number of Real Estate Properties | community	255		255
Number of apartment homes included in operating apartment communities owned | home	74,202		74,202
Basic and diluted shares outstanding
Weighted average common shares - basic | shares	133,669,584	131,330,078	132,516,847	130,165,873
Weighted average DownREIT units outstanding (in shares) | shares	7,500	7,500	7,500	7,500
Effect of dilutive securities (in shares) | shares	1,032,376	568,417	1,139,423	554,627
Weighted average common shares - diluted | shares	134,709,460	131,905,995	133,663,770	130,728,000
Calculation of Earnings per Share - basic
Net income attributable to common stockholders	$ 206,142	$ 241,100	$ 586,610	$ 540,929
Net income allocated to unvested restricted shares	(467)	(366)	(1,444)	(858)
Net income attributable to common stockholders, adjusted	$ 205,675	$ 240,734	$ 585,166	$ 540,071
Weighted average common shares - basic | shares	133,669,584	131,330,078	132,516,847	130,165,873
Earnings per common share - basic (in dollars per share) | $ / shares	$ 1.54	$ 1.83	$ 4.42	$ 4.15
Calculation of Earnings per Share - diluted
Net income attributable to common stockholders	$ 206,142	$ 241,100	$ 586,610	$ 540,929
Add: noncontrolling interests of DownREIT unitholders in consolidated partnerships, including discontinued operations	9	9	28	26
Adjusted net income available to common stockholders	$ 206,151	$ 241,109	$ 586,638	$ 540,955
Weighted average common shares - diluted | shares	134,709,460	131,905,995	133,663,770	130,728,000
Earnings per common share - diluted (in dollars per share) | $ / shares	$ 1.53	$ 1.83	$ 4.39	$ 4.14
Options to purchase shares of common stock excluded from computation of earnings per share amount (in shares) | shares				1,499
Estimated forfeiture rate of stock options (as a percent)			1.00%
Avalon at Edgewater [Member]
Loss Contingencies [Line Items]
Number of Real Estate Properties | building	2		2
Number Of Real Estate Properties, Uninhabitable | building	1		1
Number Of Units In Real Estate Property, Uninhabitable | home	240		240
Number of apartment homes included in operating apartment communities owned | home	168		168
Loss Contingency New Class Action Claims Filed Number | Claim			4
Loss Contingency Consolidated Class Action Claims | Claim			1
Loss Contingency, New Claims Filed, Number | Claim			17
Loss Contingency, Number of Plaintiffs | plaintiff			133
Loss Contingency, OSHA Citations | Citation			2
Loss Contingency, Insurance, Percentage Of Policy Amount Company Is Obligated To Fund	12.00%		12.00%
Loss Contingency, Insurance, Policy Amount For Calculating Self-Insurance Liability	$ 50,000		$ 50,000