Organization, Basis of Presentation, and Significant Accounting Policies (Details 3) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended			15 Months Ended
	Dec. 31, 2018 USD ($) community home building $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2018 USD ($) community home building $ / shares shares	Dec. 31, 2017 USD ($) land_parcel $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Mar. 31, 2016 USD ($)
Basic and diluted shares outstanding
Weighted average common shares—basic | shares									137,844,755	137,523,771	136,928,251
Weighted average DownREIT units outstanding (in shares) | shares									7,500	7,500	7,500
Effect of dilutive securities (in shares) | shares									436,986	535,415	525,886
Weighted average common shares - diluted (in shares) | shares									138,289,241	138,066,686	137,461,637
Calculation of Earnings per Share—basic
Net income attributable to common stockholders	$ 385,734	$ 192,486	$ 254,662	$ 141,643	$ 237,573	$ 238,248	$ 165,225	$ 235,875	$ 974,525	$ 876,921	$ 1,034,002
Net income allocated to unvested restricted shares									(2,839)	(2,463)	(2,610)
Net income attributable to common stockholders, adjusted									$ 971,686	$ 874,458	$ 1,031,392
Weighted average common shares—basic | shares									137,844,755	137,523,771	136,928,251
Earnings per common share - basic (in dollars per share) | $ / shares	$ 2.79	$ 1.39	$ 1.84	$ 1.03	$ 1.72	$ 1.73	$ 1.20	$ 1.72	$ 7.05	$ 6.36	$ 7.53
Calculation of Earnings per Share—diluted
Net income attributable to common stockholders	$ 385,734	$ 192,486	$ 254,662	$ 141,643	$ 237,573	$ 238,248	$ 165,225	$ 235,875	$ 974,525	$ 876,921	$ 1,034,002
Add: noncontrolling interests of DownREIT unitholders in consolidated partnerships, including discontinued operations									44	42	41
Adjusted net income attributable to common stockholders									$ 974,569	$ 876,963	$ 1,034,043
Weighted average common shares—diluted | shares									138,289,241	138,066,686	137,461,637
Earnings per common share - diluted (in dollars per share) | $ / shares	$ 2.79	$ 1.39	$ 1.84	$ 1.03	$ 1.72	$ 1.72	$ 1.20	$ 1.72	$ 7.05	$ 6.35	$ 7.52
Dividends per common share: (in dollars per share) | $ / shares									$ 5.88	$ 5.68	$ 5.4
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Abandoned pursuits costs									$ 4,388	$ 2,370	$ 4,183
Casualty and impairment loss (gain), net									215	$ 6,250	(3,935)
Number of Land Parcels Impaired | land_parcel										3
Impairment of investment in unconsolidated entities									0	$ 0	0
Business interruption insurance proceeds									$ 26	3,498	20,564
Number of Real Estate Properties | community	270								270
Number of Units in Real Estate Property | home	78,549								78,549
Land
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Casualty and impairment loss (gain), net									$ 826	9,350	10,500
Avalon at Edgewater
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Casualty and impairment loss (gain), net											(8,702)
Business interruption insurance proceeds											20,306
Insurance proceeds											29,008	$ 73,150
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 Units in Real Estate Property | home	168								168
New England
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Casualty and impairment loss (gain), net											(5,732)
Insurance recoveries											8,493
Casualty Loss											$ 2,761
Avalon Maplewood
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Casualty and impairment loss (gain), net										2,338
Business interruption insurance proceeds										3,495
Insurance recoveries										$ 19,696