Reportable Segments (Details) (USD $)	12 Months Ended			3 Months Ended							9 Months Ended
	Dec. 31, 2014 Segments	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Mar. 31, 2013	Sep. 30, 2013	Dec. 31, 2011	Dec. 31, 2009
Reportable Segment (Textual) [Abstract]
Number of reportable segments	2
Condition for Community considered to have stabilized occupancy	90% occupancy for at least three consecutive months
Number of tenants or related group of tenants that contributed 10% or more of company total revenue	0	0	0
Reportable apartment home segment rental income	$ 818,046,000	$ 758,926,000	$ 716,612,000
Reportable apartment home segment NOI	556,321,000	514,041,000	504,200,000
Joint Venture Managment and Other Fees	13,044,000	12,442,000	11,911,000
Reconciling items:
Property management	(22,142,000)	(20,780,000)	(20,465,000)
Other operating expenses	(8,271,000)	(7,136,000)	(5,718,000)
Depreciation and amortization	(358,154,000)	(341,490,000)	(350,401,000)
General and administrative	(47,800,000)	(42,238,000)	(43,792,000)
Casualty-related (recoveries)/charges, net	(541,000)	12,253,000	(8,495,000)
Other depreciation and amortization	(5,775,000)	(6,741,000)	(4,105,000)
Income/(loss) from unconsolidated entities	(7,006,000)	(415,000)	(8,579,000)
Interest Expense	(130,454,000)	(126,083,000)	(138,792,000)
Interest and Other Income Including Discontinued Operations	11,837,000	4,681,000	2,703,000
Tax benefit/(expense) of taxable REIT subsidiary, net	15,136,000	7,299,000	30,282,000
Gain Loss on the Sale of Real Estate, Including Discontinued Operations	143,647,000	41,919,000	251,554,000
Redeemable non-controlling interests in OP	(5,511,000)	(1,530,000)	(7,986,000)
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable	(3,000)	(60,000)	140,000
Net income/(loss) attributable to Entity	154,334,000	44,812,000	212,177,000
Reportable apartment home segment assets:
Total segment assets	8,383,259,000	8,207,977,000		8,383,259,000				8,207,977,000
Accumulated depreciation	(2,434,772,000)	(2,208,794,000)		(2,434,772,000)				(2,208,794,000)
Total real estate owned, net of accumulated depreciation	5,948,487,000	5,999,183,000		5,948,487,000				5,999,183,000
Reconciling items:
Cash and cash equivalents	15,224,000	30,249,000	12,115,000	15,224,000				30,249,000				12,503,000
Restricted cash	22,340,000	22,796,000		22,340,000				22,796,000
Deferred financing costs, net	22,686,000	26,924,000		22,686,000				26,924,000
Notes receivable	14,369,000	83,033,000		14,369,000				83,033,000
Redeemable Noncontrolling Interest, Equity, Common, Carrying Amount	718,226,000	507,655,000		718,226,000				507,655,000
Other assets	105,202,000	137,882,000		105,202,000				137,882,000
Total assets	6,846,534,000	6,807,722,000		6,846,534,000				6,807,722,000
Reportable Segments Additional (Textual) [Abstract]
Capital expenditures related to segments	326,461,000	452,057,000	422,480,000
Same Communities
Reportable Segments Additional (Textual) [Abstract]
Capital expenditures related to segments	55,300,000	47,500,000	46,400,000
Same Communities Western Region
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	245,803,000	231,156,000	218,268,000
Reportable apartment home segment NOI	178,926,000	166,033,000	154,205,000
Reportable apartment home segment assets:
Total segment assets	2,358,444,000	2,337,980,000		2,358,444,000				2,337,980,000
Same Communities Mid-Atlantic Region
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	161,566,000	160,208,000	155,777,000
Reportable apartment home segment NOI	111,762,000	111,643,000	108,490,000
Reportable apartment home segment assets:
Total segment assets	1,410,156,000	1,395,772,000		1,410,156,000				1,395,772,000
Same Communities Southeastern Region
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	107,991,000	102,988,000	97,699,000
Reportable apartment home segment NOI	71,528,000	67,264,000	63,122,000
Reportable apartment home segment assets:
Total segment assets	786,438,000	785,134,000		786,438,000				785,134,000
Same Communities Southwestern Region
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	54,810,000	52,302,000	46,800,000
Reportable apartment home segment NOI	33,725,000	31,927,000	27,878,000
Reportable apartment home segment assets:
Total segment assets	440,721,000	434,875,000		440,721,000				434,875,000
Same Communities Northeast Region [Member]
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	60,796,000	58,075,000	54,461,000
Reportable apartment home segment NOI	44,897,000	42,350,000	39,377,000
Reportable apartment home segment assets:
Total segment assets	746,550,000	738,805,000		746,550,000				738,805,000
Non-Mature communities/Other
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	174,183,000	150,907,000	171,239,000
Reportable apartment home segment NOI	115,483,000	94,824,000	111,128,000
Reportable apartment home segment assets:
Total segment assets	2,640,950,000	2,515,411,000		2,640,950,000				2,515,411,000
Reportable Segments Additional (Textual) [Abstract]
Capital expenditures related to segments	8,100,000	8,300,000	8,700,000
Total Communities [Member]
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	805,149,000	755,636,000	744,244,000
Segment Reconciling Items [Member]
Reconciling items:
Gain Loss on the Sale of Real Estate, Including Discontinued Operations		40,449,000
United Dominion Reality L.P.
Reportable Segment (Textual) [Abstract]
Related party management fee percentage	2.75%
Number of reportable segments	2
Condition for Community considered to have stabilized occupancy	once it achieves 90% occupancy for at least three consecutive months
Number of tenants or related group of tenants that contributed 10% or more of company total revenue	0		0
Reportable apartment home segment rental income	422,634,000	410,842,000	400,691,000
Reportable apartment home segment NOI	300,313,000	287,535,000	281,538,000
Reconciling items:
Property management	(11,622,000)	(11,298,000)	(11,019,000)
Other operating expenses	(5,172,000)	(5,728,000)	(5,272,000)
Depreciation and amortization	(179,176,000)	(181,302,000)	(195,051,000)
Interest	(41,717,000)	(36,058,000)	(45,234,000)
General and administrative	(28,541,000)	(24,808,000)	(26,204,000)
Casualty-related (recoveries)/charges, net	(541,000)	8,083,000	(5,518,000)
Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Nonredeemable	952,000	4,566,000	352,000
Net (loss)/gain on sale of depreciable property	63,635,000	41,518,000	51,094,000
Net income/(loss) attributable to Entity	96,227,000	73,376,000	43,982,000	32,631,000	8,637,000	24,426,000	30,533,000	44,482,000	10,154,000	7,729,000	11,011,000
Reportable apartment home segment assets:
Total segment assets	4,238,770,000	4,188,480,000		4,238,770,000				4,188,480,000
Accumulated depreciation	(1,403,303,000)	(1,241,574,000)		(1,403,303,000)				(1,241,574,000)
Total real estate owned, net of accumulated depreciation	2,835,467,000	2,946,906,000		2,835,467,000				2,946,906,000
Reconciling items:
Cash and cash equivalents	502,000	1,897,000	2,804,000	502,000				1,897,000					704,000
Restricted cash	13,811,000	13,526,000		13,811,000				13,526,000
Deferred financing costs, net	4,475,000	5,848,000		4,475,000				5,848,000
Other assets	24,029,000	25,064,000		24,029,000				25,064,000
Total assets	2,878,284,000	2,993,241,000		2,878,284,000				2,993,241,000
Reportable Segments Additional (Textual) [Abstract]
Capital expenditures related to segments	91,682,000	151,002,000	115,355,000
United Dominion Reality L.P. | Same Communities
Reportable Segments Additional (Textual) [Abstract]
Capital expenditures related to segments	32,800,000	25,800,000
United Dominion Reality L.P. | Same Communities Western Region
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	194,105,000	181,935,000	172,096,000
Reportable apartment home segment NOI	142,388,000	131,276,000	122,477,000
Reportable apartment home segment assets:
Total segment assets	1,749,494,000	1,733,144,000		1,749,494,000				1,733,144,000
United Dominion Reality L.P. | Same Communities Mid-Atlantic Region
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	68,822,000	68,205,000	66,487,000
Reportable apartment home segment NOI	46,639,000	46,770,000	45,801,000
Reportable apartment home segment assets:
Total segment assets	713,093,000	706,447,000		713,093,000				706,447,000
United Dominion Reality L.P. | Same Communities Southeastern Region
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	45,224,000	43,208,000	40,771,000
Reportable apartment home segment NOI	29,815,000	28,105,000	26,510,000
Reportable apartment home segment assets:
Total segment assets	333,428,000	328,150,000		333,428,000				328,150,000
United Dominion Reality L.P. | Same Communities Southwestern Region
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	26,580,000	25,614,000	23,980,000
Reportable apartment home segment NOI	16,821,000	16,057,000	14,738,000
Reportable apartment home segment assets:
Total segment assets	228,996,000	226,252,000		228,996,000				226,252,000
United Dominion Reality L.P. | Same Communities Northeast Region [Member]
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	38,087,000	36,623,000	34,579,000
Reportable apartment home segment NOI	28,930,000	27,149,000	25,653,000
Reportable apartment home segment assets:
Total segment assets	447,269,000	443,483,000		447,269,000				443,483,000
United Dominion Reality L.P. | Non-Mature communities/Other
Reportable Segment (Textual) [Abstract]
Reportable apartment home segment rental income	49,816,000	55,257,000	62,778,000
Reportable apartment home segment NOI	35,720,000	38,178,000	46,359,000
Reportable apartment home segment assets:
Total segment assets	766,490,000	751,004,000		766,490,000				751,004,000
Reportable Segments Additional (Textual) [Abstract]
Capital expenditures related to segments	$ 1,000,000	$ 2,000,000
United Dominion Reality L.P. | Minimum
Reportable Segment (Textual) [Abstract]
Percent occupancy to be considered a community	90.00%
Time to maintain percent occupancy to be considered a community	3 months
United Dominion Reality L.P. | Taxable REIT Subsidiaries
Reportable Segment (Textual) [Abstract]
Related party management fee percentage	2.75%