Organization, Basis of Presentation, and Significant Accounting Policies (Details 3) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) community home $ / shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($) community home $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Basic and diluted shares outstanding
Weighted average common shares—basic | shares									139,054,191	137,844,755	137,523,771
Weighted average DownREIT units outstanding (in shares) | shares									7,500	7,500	7,500
Effect of dilutive securities (in shares) | shares									509,859	436,986	535,415
Weighted average common shares - diluted (in shares) | shares									139,571,550	138,289,241	138,066,686
Calculation of Earnings per Share—basic
Net income attributable to common stockholders	$ 167,650	$ 279,677	$ 168,281	$ 170,366	$ 385,734	$ 192,486	$ 254,662	$ 141,643	$ 785,974	$ 974,525	$ 876,921
Net income allocated to unvested restricted shares									(2,063)	(2,839)	(2,463)
Net income attributable to common stockholders, adjusted									$ 783,911	$ 971,686	$ 874,458
Weighted average common shares—basic | shares									139,054,191	137,844,755	137,523,771
Earnings per common share - basic (in dollars per share) | $ / shares	$ 1.20	$ 2.00	$ 1.21	$ 1.23	$ 2.79	$ 1.39	$ 1.84	$ 1.03	$ 5.64	$ 7.05	$ 6.36
Calculation of Earnings per Share—diluted
Net income attributable to common stockholders	$ 167,650	$ 279,677	$ 168,281	$ 170,366	$ 385,734	$ 192,486	$ 254,662	$ 141,643	$ 785,974	$ 974,525	$ 876,921
Add: noncontrolling interests of DownREIT unitholders in consolidated partnerships, including discontinued operations									46	44	42
Adjusted net income attributable to common stockholders									$ 786,020	$ 974,569	$ 876,963
Weighted average common shares—diluted | shares									139,571,550	138,289,241	138,066,686
Earnings per common share - diluted (in dollars per share) | $ / shares	$ 1.20	$ 2.00	$ 1.21	$ 1.23	$ 2.79	$ 1.39	$ 1.84	$ 1.03	$ 5.63	$ 7.05	$ 6.35
Real Estate [Line Items]
Operating Lease, Lease Income									$ 2,319,666	$ 2,280,963	$ 2,154,481
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Abandoned pursuits costs									4,896	4,388	2,370
Casualty and impairment loss, net									0	215	6,250
Impairment of investment in unconsolidated entities									0	0	0
Management, development and other fees									4,960	3,572	4,147
Business interruption insurance proceeds									1,441	26	3,498
Revenues	$ 593,566	$ 587,613	$ 577,263	$ 566,184	$ 578,522	$ 575,982	$ 569,239	$ 560,792	$ 2,324,626	2,284,535	2,158,628
Number of real estate properties | community	275								275
Number of Units in Real Estate Property | home	79,886								79,886
Land
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Casualty and impairment loss, net									$ 0	826	9,350
Avalon Maplewood
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Casualty and impairment loss, net										2,338
Business interruption insurance proceeds										3,495
Insurance recoveries										19,696
Continuing Operations
Real Estate [Line Items]
Operating Lease, Lease Income									2,287,927	2,136,246	1,974,932
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									13,817	9,818	10,026
Business interruption insurance proceeds									1,441	26	3,498
Revenues									2,303,185	2,146,090	1,988,456
Management, Development And Other [Member] | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									4,960	3,572	4,147
Rental and non-rental related income | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									8,857	6,246	5,879
Corporate, Non-Segment [Member]
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Revenues									4,960	3,572	4,147
Corporate, Non-Segment [Member] | Continuing Operations
Real Estate [Line Items]
Operating Lease, Lease Income									0	0	0
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									4,960	3,572	4,147
Business interruption insurance proceeds									0	0	0
Revenues									4,960	3,572	4,147
Corporate, Non-Segment [Member] | Management, Development And Other [Member] | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									4,960	3,572	4,147
Corporate, Non-Segment [Member] | Rental and non-rental related income | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									0	0	0
Established Communities | Operating segment
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Revenues									1,836,339	1,783,112	1,574,110
Established Communities | Operating segment | New England
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Revenues									249,301	241,793	215,133
Established Communities | Operating segment | Continuing Operations
Real Estate [Line Items]
Operating Lease, Lease Income									1,829,748	1,778,841	1,570,262
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									6,113	4,245	3,845
Business interruption insurance proceeds									478	26	3
Revenues									1,836,339	1,783,112	1,574,110
Established Communities | Operating segment | Management, Development And Other [Member] | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									0	0	0
Established Communities | Operating segment | Rental and non-rental related income | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									6,113	4,245	3,845
Other Stabilized Communities | Operating segment
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Revenues									298,415	238,584	174,933
Other Stabilized Communities | Operating segment | Continuing Operations
Real Estate [Line Items]
Operating Lease, Lease Income									295,511	236,852	173,639
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									1,941	1,732	1,294
Business interruption insurance proceeds									963	0	0
Revenues									298,415	238,584	174,933
Other Stabilized Communities | Operating segment | Management, Development And Other [Member] | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									0	0	0
Other Stabilized Communities | Operating segment | Rental and non-rental related income | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									1,941	1,732	1,294
Development/Redevelopment Communities | Operating segment | Continuing Operations
Real Estate [Line Items]
Operating Lease, Lease Income									162,668	120,553	231,031
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									803	269	740
Business interruption insurance proceeds									0	0	3,495
Revenues									163,471	120,822	235,266
Development/Redevelopment Communities | Operating segment | Management, Development And Other [Member] | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									0	0	0
Development/Redevelopment Communities | Operating segment | Rental and non-rental related income | Continuing Operations
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Management, development and other fees									$ 803	$ 269	$ 740