Schedule III - Real Estate and Accumulated Depreciation (Details) (USD $)																																																																					12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013 Multifamily Properties [Member]	Dec. 31, 2013 Mallard's Crossing [Member]		Dec. 31, 2013 The Residence at Barrington [Member]		Dec. 31, 2013 Village at Avon [Member]		Dec. 31, 2013 Village of Western Reserve [Member]		Dec. 31, 2013 Westchester Townhomes [Member]		Dec. 31, 2013 Westlake Seven [Member]		Dec. 31, 2013 Williamsburg Townhomes [Member]		Dec. 31, 2013 Bedford Commons [Member]		Dec. 31, 2013 Heathermoor [Member]		Dec. 31, 2013 Kensington Grove [Member]		Dec. 31, 2013 Lake Forest [Member]		Dec. 31, 2013 Perimeter Lakes [Member]		Dec. 31, 2013 Saw Mill Village [Member]		Dec. 31, 2013 Sterling Park [Member]		Dec. 31, 2013 The Residence at Christopher Wren [Member]		Dec. 31, 2013 Wyndemere Land [Member]		Dec. 31, 2013 Arbor Landings [Member]		Dec. 31, 2013 Clinton Place [Member]		Dec. 31, 2013 Georgetown Park [Member]		Dec. 31, 2013 Oaks at Hampton [Member]		Dec. 31, 2013 Spring Valley [Member]		Dec. 31, 2013 Landings at the Preserve [Member]		Dec. 31, 2013 Summer Ridge [Member]		Dec. 31, 2013 Cypress Shores [Member]		Dec. 31, 2013 Doral West [Member]		Dec. 31, 2013 Vista Lago [Member]		Dec. 31, 2013 Waterstone at Wellington [Member]		Dec. 31, 2013 Windsor Pines [Member]		Dec. 31, 2013 Cambridge at Buckhead [Member]		Dec. 31, 2013 Morgan Place [Member]		Dec. 31, 2013 Annen Woods [Member]		Dec. 31, 2013 Reflections [Member]		Dec. 31, 2013 7001 Arlington Road [Member]		Dec. 31, 2013 Dwell Vienna Metro [Member]		Dec. 31, 2013 Hampton Point [Member]		Dec. 31, 2013 Center Point [Member]		Dec. 31, 2013 Residence at White River [Member]		Dec. 31, 2013 Steeplechase at Shiloh [Member]		Dec. 31, 2013 Lofts at Weston Lakeside [Member]		Dec. 31, 2013 Southpoint Village [Member]		Dec. 31, 2013 St. Mary's Square [Member]		Dec. 31, 2013 The Apartments at the Arboretum [Member]		Dec. 31, 2013 The Park at Crossroads [Member]		Dec. 31, 2013 The Apartments at Blakeney [Member]		Dec. 31, 2013 Ashborough [Member]		Dec. 31, 2013 Riverside Station [Member]		Dec. 31, 2013 Westwind Farms [Member]		Dec. 31, 2013 River Forest [Member]		Dec. 31, 2013 The Alexander at Ghent [Member]		Dec. 31, 2013 The Belvedere [Member]		Dec. 31, 2013 350 Eighth [Member]		Dec. 31, 2013 Desmond's Tower [Member]		Dec. 31, 2013 The Desmond on Wilshire [Member]		Dec. 31, 2013 Cantabria [Member]		Dec. 31, 2013 Rienzi at Turtle Creek [Member]		Dec. 31, 2013 San Raphael [Member]		Dec. 31, 2013 San Raphael Phase II [Member]		Dec. 31, 2013 The Brixton [Member]		Dec. 31, 2013 21-Forty Medical District [Member]		Dec. 31, 2013 Vista Germantown [Member]		Dec. 31, 2013 Miscellaneous [Member]	Dec. 31, 2013 Land, Buildings and Improvements [Member]	Dec. 31, 2013 Minimum [Member] Mallard's Crossing [Member]	Dec. 31, 2013 Minimum [Member] The Residence at Barrington [Member]	Dec. 31, 2013 Minimum [Member] Village at Avon [Member]	Dec. 31, 2013 Minimum [Member] Village of Western Reserve [Member]	Dec. 31, 2013 Minimum [Member] Westchester Townhomes [Member]	Dec. 31, 2013 Minimum [Member] Westlake Seven [Member]	Dec. 31, 2013 Minimum [Member] Williamsburg Townhomes [Member]	Dec. 31, 2013 Minimum [Member] Bedford Commons [Member]	Dec. 31, 2013 Minimum [Member] Heathermoor [Member]	Dec. 31, 2013 Minimum [Member] Kensington Grove [Member]	Dec. 31, 2013 Minimum [Member] Lake Forest [Member]	Dec. 31, 2013 Minimum [Member] Perimeter Lakes [Member]	Dec. 31, 2013 Minimum [Member] Saw Mill Village [Member]	Dec. 31, 2013 Minimum [Member] Sterling Park [Member]	Dec. 31, 2013 Minimum [Member] The Residence at Christopher Wren [Member]	Dec. 31, 2013 Minimum [Member] Arbor Landings [Member]	Dec. 31, 2013 Minimum [Member] Clinton Place [Member]	Dec. 31, 2013 Minimum [Member] Georgetown Park [Member]	Dec. 31, 2013 Minimum [Member] Oaks at Hampton [Member]	Dec. 31, 2013 Minimum [Member] Spring Valley [Member]	Dec. 31, 2013 Minimum [Member] Landings at the Preserve [Member]	Dec. 31, 2013 Minimum [Member] Summer Ridge [Member]	Dec. 31, 2013 Minimum [Member] Cypress Shores [Member]	Dec. 31, 2013 Minimum [Member] Vista Lago [Member]	Dec. 31, 2013 Minimum [Member] Waterstone at Wellington [Member]	Dec. 31, 2013 Minimum [Member] Windsor Pines [Member]	Dec. 31, 2013 Minimum [Member] Cambridge at Buckhead [Member]	Dec. 31, 2013 Minimum [Member] Morgan Place [Member]	Dec. 31, 2013 Minimum [Member] Annen Woods [Member]	Dec. 31, 2013 Minimum [Member] Reflections [Member]	Dec. 31, 2013 Minimum [Member] Dwell Vienna Metro [Member]	Dec. 31, 2013 Minimum [Member] Hampton Point [Member]	Dec. 31, 2013 Minimum [Member] Center Point [Member]	Dec. 31, 2013 Minimum [Member] Residence at White River [Member]	Dec. 31, 2013 Minimum [Member] Steeplechase at Shiloh [Member]	Dec. 31, 2013 Minimum [Member] Southpoint Village [Member]	Dec. 31, 2013 Minimum [Member] The Apartments at the Arboretum [Member]	Dec. 31, 2013 Minimum [Member] The Park at Crossroads [Member]	Dec. 31, 2013 Minimum [Member] Ashborough [Member]	Dec. 31, 2013 Minimum [Member] Riverside Station [Member]	Dec. 31, 2013 Minimum [Member] Westwind Farms [Member]	Dec. 31, 2013 Minimum [Member] River Forest [Member]	Dec. 31, 2013 Minimum [Member] The Alexander at Ghent [Member]	Dec. 31, 2013 Minimum [Member] The Belvedere [Member]	Dec. 31, 2013 Minimum [Member] Desmond's Tower [Member]	Dec. 31, 2013 Minimum [Member] San Raphael [Member]	Dec. 31, 2013 Minimum [Member] San Raphael Phase II [Member]	Dec. 31, 2013 Minimum [Member] The Brixton [Member]	Dec. 31, 2013 Minimum [Member] 21-Forty Medical District [Member]	Dec. 31, 2013 Minimum [Member] Vista Germantown [Member]	Dec. 31, 2013 Minimum [Member] Miscellaneous [Member]	Dec. 31, 2013 Maximum [Member] Mallard's Crossing [Member]	Dec. 31, 2013 Maximum [Member] The Residence at Barrington [Member]	Dec. 31, 2013 Maximum [Member] Village at Avon [Member]	Dec. 31, 2013 Maximum [Member] Village of Western Reserve [Member]	Dec. 31, 2013 Maximum [Member] Westchester Townhomes [Member]	Dec. 31, 2013 Maximum [Member] Westlake Seven [Member]	Dec. 31, 2013 Maximum [Member] Williamsburg Townhomes [Member]	Dec. 31, 2013 Maximum [Member] Bedford Commons [Member]	Dec. 31, 2013 Maximum [Member] Heathermoor [Member]	Dec. 31, 2013 Maximum [Member] Kensington Grove [Member]	Dec. 31, 2013 Maximum [Member] Lake Forest [Member]	Dec. 31, 2013 Maximum [Member] Perimeter Lakes [Member]	Dec. 31, 2013 Maximum [Member] Saw Mill Village [Member]	Dec. 31, 2013 Maximum [Member] Sterling Park [Member]	Dec. 31, 2013 Maximum [Member] The Residence at Christopher Wren [Member]	Dec. 31, 2013 Maximum [Member] Arbor Landings [Member]	Dec. 31, 2013 Maximum [Member] Clinton Place [Member]	Dec. 31, 2013 Maximum [Member] Georgetown Park [Member]	Dec. 31, 2013 Maximum [Member] Oaks at Hampton [Member]	Dec. 31, 2013 Maximum [Member] Spring Valley [Member]	Dec. 31, 2013 Maximum [Member] Landings at the Preserve [Member]	Dec. 31, 2013 Maximum [Member] Summer Ridge [Member]	Dec. 31, 2013 Maximum [Member] Cypress Shores [Member]	Dec. 31, 2013 Maximum [Member] Vista Lago [Member]	Dec. 31, 2013 Maximum [Member] Waterstone at Wellington [Member]	Dec. 31, 2013 Maximum [Member] Windsor Pines [Member]	Dec. 31, 2013 Maximum [Member] Cambridge at Buckhead [Member]	Dec. 31, 2013 Maximum [Member] Morgan Place [Member]	Dec. 31, 2013 Maximum [Member] Annen Woods [Member]	Dec. 31, 2013 Maximum [Member] Reflections [Member]	Dec. 31, 2013 Maximum [Member] Dwell Vienna Metro [Member]	Dec. 31, 2013 Maximum [Member] Hampton Point [Member]	Dec. 31, 2013 Maximum [Member] Center Point [Member]	Dec. 31, 2013 Maximum [Member] Residence at White River [Member]	Dec. 31, 2013 Maximum [Member] Steeplechase at Shiloh [Member]	Dec. 31, 2013 Maximum [Member] Southpoint Village [Member]	Dec. 31, 2013 Maximum [Member] The Apartments at the Arboretum [Member]	Dec. 31, 2013 Maximum [Member] The Park at Crossroads [Member]	Dec. 31, 2013 Maximum [Member] Ashborough [Member]	Dec. 31, 2013 Maximum [Member] Riverside Station [Member]	Dec. 31, 2013 Maximum [Member] Westwind Farms [Member]	Dec. 31, 2013 Maximum [Member] River Forest [Member]	Dec. 31, 2013 Maximum [Member] The Alexander at Ghent [Member]	Dec. 31, 2013 Maximum [Member] The Belvedere [Member]	Dec. 31, 2013 Maximum [Member] Desmond's Tower [Member]	Dec. 31, 2013 Maximum [Member] San Raphael [Member]	Dec. 31, 2013 Maximum [Member] San Raphael Phase II [Member]	Dec. 31, 2013 Maximum [Member] The Brixton [Member]	Dec. 31, 2013 Maximum [Member] 21-Forty Medical District [Member]	Dec. 31, 2013 Maximum [Member] Vista Germantown [Member]	Dec. 31, 2013 Maximum [Member] Miscellaneous [Member]	Dec. 31, 2013 Furniture and Fixtures [Member]	Dec. 31, 2013 Desmond's Tower [Member] sqft
Real Estate and Accumulated Depreciation [Line Items]
Depreciable Lives - Years																																																																																																																																5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	5 years	14 years	5 years	5 years	5 years	5 years	5 years	10 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years	30 years
Land, Cost					$ 296,583,000	$ 941,000		$ 2,357,000		$ 2,145,000		$ 691,000		$ 693,000		$ 559,000		$ 844,000		$ 929,000		$ 1,796,000		$ 533,000		$ 824,000		$ 1,265,000		$ 2,548,000		$ 646,000		$ 1,560,000		$ 200,000		$ 1,129,000		$ 1,219,000		$ 1,778,000		$ 3,026,000		$ 1,433,000		$ 1,081,000		$ 1,251,000		$ 2,769,000		$ 12,060,000	[1]	$ 4,012,000	[1]	$ 5,948,000	[1]	$ 4,834,000		$ 6,166,000	[1]	$ 3,292,000		$ 1,389,000		$ 1,807,000		$ 12,650,000	[2]	$ 11,241,000	[1]	$ 3,394,000		$ 1,508,000		$ 1,064,000		$ 2,261,000		$ 4,112,000	[1]	$ 3,307,000	[1]	$ 3,065,000	[1]	$ 4,008,000	[1]	$ 5,520,000	[1]	$ 5,430,000	[1]	$ 20,136,000	[1]	$ 13,680,000	[1]	$ 18,265,000	[1]	$ 4,655,000	[1]	$ 4,368,000	[1]	$ 6,044,000	[1]	$ 45,936,000	[2]	$ 7,390,000	[1],[3]	$ 17,491,000	[2]	$ 6,983,000	[2]	$ 3,911,000	[1]	$ 3,777,000	[1]	$ 733,000	[2]	$ 2,867,000	[1]	$ 5,413,000	[1]	$ 5,649,000
Buildings & Improvements, Cost					1,258,036,000	8,499,000		21,986,000		21,704,000		6,866,000		5,686,000		332,000		12,787,000		5,751,000		8,535,000		4,600,000		6,135,000		8,647,000		17,218,000		3,919,000		13,754,000		0		10,403,000		9,478,000		12,141,000		27,204,000		13,461,000		7,190,000		11,194,000		16,452,000		79,513,000	[1]	35,954,000	[1]	25,456,000	[1]	28,795,000		16,730,000	[1]	9,159,000		9,069,000		12,447,000		0	[2]	67,130,000	[1]	21,703,000		22,861,000		11,631,000		16,257,000		33,105,000	[1]	29,661,000	[1]	23,481,000	[1]	33,411,000	[1]	31,391,000	[1]	46,013,000	[1]	65,885,000	[1]	38,708,000	[1]	67,098,000	[1]	25,758,000	[1]	43,263,000	[1]	35,353,000	[1]	0	[2]	10,615,000	[1],[3]	0	[2]	0	[2]	43,680,000	[1]	15,795,000	[1]	12,322,000	[2]	17,638,000	[1]	44,823,000	[1]	29,389,000
Costs Capitalized Subsequent to Acquisition	114,382,000				107,461,000	2,724,000		1,804,000		2,742,000		732,000		1,157,000		345,000		3,755,000		1,033,000		2,132,000		905,000		1,034,000		1,483,000		4,440,000		588,000		3,813,000		0		10,986,000		2,383,000		14,116,000		4,755,000		1,882,000		1,380,000		2,584,000		1,965,000		405,000	[1]	588,000	[1]	551,000	[1]	1,669,000		979,000	[1]	1,322,000		1,983,000		4,018,000		0	[2]	423,000	[1]	7,237,000		2,298,000		2,387,000		1,087,000		0	[1]	251,000	[1]	0	[1]	339,000	[1]	615,000	[1]	28,000	[1]	579,000	[1]	419,000	[1]	593,000	[1]	7,252,000	[1]	1,045,000	[1]	465,000	[1]	0	[2]	0	[1],[3]	0	[2]	0	[2]	30,000	[1]	900,000	[1]	0	[2]	618,000	[1]	446,000	[1]	196,000		6,921,000
Land, Gross					296,937,000	941,000		2,355,000		2,145,000		691,000		693,000		0		844,000		929,000		1,796,000		533,000		824,000		1,265,000		2,548,000		646,000		1,560,000		200,000		1,682,000		1,219,000		2,128,000		3,026,000		1,433,000		1,143,000		1,251,000		2,769,000		12,060,000	[1]	4,012,000	[1]	5,948,000	[1]	4,834,000		6,166,000	[1]	3,292,000		1,389,000		1,807,000		12,650,000	[2]	11,241,000	[1]	3,394,000		1,470,000		1,064,000		2,261,000		4,112,000	[1]	3,307,000	[1]	3,065,000	[1]	4,008,000	[1]	5,520,000	[1]	5,430,000	[1]	20,136,000	[1]	13,680,000	[1]	18,265,000	[1]	4,644,000	[1]	4,367,000	[1]	6,044,000	[1]	45,936,000	[2]	7,390,000	[1],[3]	17,491,000	[2]	6,983,000	[2]	3,911,000	[1]	3,777,000	[1]	733,000	[2]	2,867,000	[1]	5,413,000	[1]	5,649,000		1,504,000	298,441,000
Buildings & Improvements, Gross					1,365,143,000	11,223,000		23,792,000		24,446,000		7,598,000		6,843,000		1,236,000		16,542,000		6,784,000		10,667,000		5,505,000		7,169,000		10,130,000		21,658,000		4,507,000		17,567,000		0		20,836,000		11,861,000		25,907,000		31,959,000		15,343,000		8,508,000		13,778,000		18,417,000		79,918,000	[1]	36,542,000	[1]	26,007,000	[1]	30,464,000		17,709,000	[1]	10,481,000		11,052,000		16,465,000		0	[2]	67,553,000	[1]	28,940,000		25,197,000		14,018,000		17,344,000		33,105,000	[1]	29,912,000	[1]	23,481,000	[1]	33,750,000	[1]	32,006,000	[1]	46,041,000	[1]	66,464,000	[1]	39,127,000	[1]	67,691,000	[1]	33,021,000	[1]	44,309,000	[1]	35,818,000	[1]	0	[2]	10,615,000	[1],[3]	0	[2]	0	[2]	43,710,000	[1]	16,695,000	[1]	12,322,000	[2]	18,256,000	[1]	45,269,000	[1]	29,585,000		5,417,000	1,370,560,000
Total, Gross	1,751,519,000	1,511,647,000	1,345,439,000	1,210,289,000	1,662,080,000	12,164,000		26,147,000		26,591,000		8,289,000		7,536,000		1,236,000		17,386,000		7,713,000		12,463,000		6,038,000		7,993,000		11,395,000		24,206,000		5,153,000		19,127,000		200,000		22,518,000		13,080,000		28,035,000		34,985,000		16,776,000		9,651,000		15,029,000		21,186,000		91,978,000	[1]	40,554,000	[1]	31,955,000	[1]	35,298,000		23,875,000	[1]	13,773,000		12,441,000		18,272,000		12,650,000	[2]	78,794,000	[1]	32,334,000		26,667,000		15,082,000		19,605,000		37,217,000	[1]	33,219,000	[1]	26,546,000	[1]	37,758,000	[1]	37,526,000	[1]	51,471,000	[1]	86,600,000	[1]	52,807,000	[1]	85,956,000	[1]	37,665,000	[1]	48,676,000	[1]	41,862,000	[1]	45,936,000	[2]	18,005,000	[1],[3]	17,491,000	[2]	6,983,000	[2]	47,621,000	[1]	20,472,000	[1]	13,055,000	[2]	21,123,000	[1]	50,682,000	[1]	35,234,000		6,921,000	1,669,001,000
Accumulated Depreciation, Gross	386,841,000	371,730,000	358,605,000	335,289,000	358,636,000	7,003,000		12,349,000		11,323,000		3,772,000		5,658,000		1,152,000		10,595,000		4,221,000		6,858,000		3,309,000		4,512,000		6,001,000		12,143,000		2,839,000		11,127,000		0		11,493,000		6,519,000		15,259,000		19,587,000		8,433,000		5,065,000		8,004,000		9,758,000		1,466,000	[1]	11,706,000	[1]	2,698,000	[1]	15,729,000		5,325,000	[1]	5,385,000		5,735,000		8,439,000		0	[2]	6,210,000	[1]	14,765,000		13,817,000		7,614,000		8,937,000		122,000	[1]	1,699,000	[1]	81,000	[1]	2,202,000	[1]	1,898,000	[1]	487,000	[1]	8,426,000	[1]	5,499,000	[1]	8,186,000	[1]	6,725,000	[1]	10,982,000	[1]	7,602,000	[1]	0	[2]	714,000	[1],[3]	0	[2]	0	[2]	413,000	[1]	2,233,000	[1]	136,000	[2]	1,633,000	[1]	2,635,000	[1]	2,157,000		2,996,000	361,632,000
Encumbrances					279,474,000	0	[4]	0	[4]	19,945,000	[4]	0	[4]	0	[4]	0	[4]	0	[4]	0	[4]	0	[4]	0	[4]	0	[4]	0	[4]	16,191,000	[4]	0	[4]	0	[4]	0	[4]	0	[4]	0	[4]	0	[4]	0	[4]	10,817,000	[4]	0	[4]	0	[4]	0	[4]	0	[1],[4]	0	[1],[4]	0	[1],[4]	0	[4]	0	[2]	0	[4]	0	[4]	0	[4]	2,182,000	[2],[4]	41,167,000	[1],[4]	0	[4]	0	[4]	9,221,000	[4]	0	[4]	0	[1],[4]	0	[1],[4]	0	[1],[4]	0	[1],[4]	26,249,000	[1],[4]	28,000,000	[1],[4]	47,591,000	[1],[4]	35,380,000	[1],[4]	0	[1],[4]	0	[1],[4]	24,500,000	[1],[4]	0	[1],[4]	0	[2],[4]	0	[1],[3],[4]	0	[2],[4]	5,918,000	[2],[4]	0	[1],[4]	0	[1],[4]	0	[2],[4]	12,313,000	[1],[4]	0	[1],[4]	0	[4]
Furniture and fixtures	39,725,000	36,997,000
Construction in progress	42,793,000	10,449,000
Real Estate Investment Property, at Cost	1,751,519,000	1,511,647,000
Real Estate Investment Property, Accumulated Depreciation	386,841,000	371,730,000																																																																																																																																																																																																																																				25,209,000
Investment in unconsolidated entities	9,321,000	0
Square feet of space																																																																																																																																																																																																																																							78,800
Aggregate cost for federal income tax purposes	$ 1,600,000,000

[1] For acquisitions initiated after June 30, 2001, intangible assets such as in place leases were recognized upon the acquisition of these properties and are not included in the initial costs or historical costs shown.

[2] Currently under development.

[3] Commercial building containing 78,800 total square feet (unaudited).

[4] Encumbrances include mortgage debt secured by the real estate assets.