Organization, Basis of Presentation and Significant Accounting Policies (Details 2) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) community Claim home building $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) community Claim home plaintiff building $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares
Loss Contingencies [Line Items]
Number of Real Estate Properties | community	260		260
Number of apartment homes included in operating apartment communities owned | home	75,504		75,504
Basic and diluted shares outstanding
Weighted average common shares - basic | shares	136,918,770	131,977,578	136,852,323	131,930,916
Weighted average DownREIT units outstanding (in shares) | shares	7,500	7,500	7,500	7,500
Effect of dilutive securities (in shares) | shares	511,463	1,101,361	550,564	1,192,947
Weighted average common shares - diluted | shares	137,437,733	133,086,439	137,410,387	133,131,363
Calculation of Earnings per Share - basic
Net income attributable to common stockholders | $	$ 197,444	$ 172,324	$ 435,377	$ 380,469
Net income allocated to unvested restricted shares | $	(516)	(445)	(1,147)	(975)
Net income attributable to common stockholders, adjusted | $	$ 196,928	$ 171,879	$ 434,230	$ 379,494
Weighted average common shares - basic | shares	136,918,770	131,977,578	136,852,323	131,930,916
Earnings per common share - basic (in dollars per share) | $ / shares	$ 1.44	$ 1.30	$ 3.17	$ 2.88
Calculation of Earnings per Share - diluted
Net income attributable to common stockholders | $	$ 197,444	$ 172,324	$ 435,377	$ 380,469
Add: noncontrolling interests of DownREIT unitholders in consolidated partnerships, including discontinued operations | $	10	9	20	19
Adjusted net income available to common stockholders | $	$ 197,454	$ 172,333	$ 435,397	$ 380,488
Weighted average common shares - diluted | shares	137,437,733	133,086,439	137,410,387	133,131,363
Earnings per common share - diluted (in dollars per share) | $ / shares	$ 1.44	$ 1.29	$ 3.17	$ 2.86
Estimated forfeiture rate of stock options (as a percent)			0.80%
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			3
Loss Contingency, New Claims Filed, Number | Claim			20
Loss Contingency, Number of Plaintiffs | plaintiff			141
Loss Contingency, Pending Claims, Number | Claim	19		19