Acquisitions And Development (Property Acquisitions) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Property Acquisitions [Line Items]
Number of Units	41,951
Cost of Acquisition	$ 511,308		$ 346,770
Middlebrooke Apartments [Member]
Property Acquisitions [Line Items]
Apartment Community	Middlebrooke Apartments
Market Area	Baltimore
Date Acquired	4/1/2010
Year Constructed	1974-1977
Number of Units	208
Cost of Acquisition	17,350
Cost of Acquisition Per Unit	83
Westbrooke Apartments [Member]
Property Acquisitions [Line Items]
Apartment Community	Westbrooke Apartments
Market Area	Baltimore
Date Acquired	4/1/2010
Year Constructed	1961-1970
Number of Units	110
Cost of Acquisition	6,350
Cost of Acquisition Per Unit	58
Annapolis Roads Apartments [Member]
Property Acquisitions [Line Items]
Apartment Community	Annapolis Roads Apartments(1)	[1]
Market Area	Baltimore	[1]
Date Acquired	6/17/2010	[1]
Year Constructed	1974-1979	[1]
Number of Units	282	[1]
Cost of Acquisition	32,500	[1]
Cost of Acquisition Per Unit	115	[1]
The Greens At Columbia [Member]
Property Acquisitions [Line Items]
Apartment Community	The Greens at Columbia
Market Area	Baltimore
Date Acquired	7/29/2010
Year Constructed	1986-1987
Number of Units	168
Cost of Acquisition	25,600
Cost of Acquisition Per Unit	152
Village At Potomac Falls [Member]
Property Acquisitions [Line Items]
Apartment Community	Village at Potomac Falls
Market Area	Suburban D.C.
Date Acquired	8/5/2010
Year Constructed	1999
Number of Units	247
Cost of Acquisition	38,500
Cost of Acquisition Per Unit	156
Charleston Place [Member]
Property Acquisitions [Line Items]
Apartment Community	Charleston Place
Market Area	Baltimore
Date Acquired	9/30/2010
Year Constructed	1971-1983
Number of Units	858
Cost of Acquisition	103,000
Cost of Acquisition Per Unit	120
The Courts At Fair Oaks [Member]
Property Acquisitions [Line Items]
Apartment Community	The Courts at Fair Oaks
Market Area	Suburban D.C.
Date Acquired	9/30/2010
Year Constructed	1990
Number of Units	364
Cost of Acquisition	70,100
Cost of Acquisition Per Unit	193
Crescent Club Apartments [Member]
Property Acquisitions [Line Items]
Apartment Community	Crescent Club Apartments
Market Area	Long Island
Date Acquired	9/30/2010
Year Constructed	1973
Number of Units	257
Cost of Acquisition	31,250
Cost of Acquisition Per Unit	122
Lakeview Townhomes [Member]
Property Acquisitions [Line Items]
Apartment Community	Lakeview Townhomes
Market Area	Chicago
Date Acquired	10/18/2010
Year Constructed	1996
Number of Units	120
Cost of Acquisition	14,475
Cost of Acquisition Per Unit	121
Hunters Glen [Member]
Property Acquisitions [Line Items]
Apartment Community	Hunters Glen
Market Area	Suburban D.C.
Date Acquired	4/19/2011
Year Constructed	1984
Number of Units	108
Cost of Acquisition	7,000
Cost of Acquisition Per Unit	65
Waterview Apartments [Member]
Property Acquisitions [Line Items]
Apartment Community	Waterview Apartments
Market Area	Philadelphia
Date Acquired	7/14/2011
Year Constructed	1968
Number of Units	203
Cost of Acquisition	24,600
Cost of Acquisition Per Unit	121
The Commons At Haynes Farm [Member]
Property Acquisitions [Line Items]
Apartment Community	The Commons at Haynes Farm
Market Area	Boston
Date Acquired	7/15/2011
Year Constructed	1991
Number of Units	302
Cost of Acquisition	40,500
Cost of Acquisition Per Unit	134
The Apts At Cambridge Court [Member]
Property Acquisitions [Line Items]
Apartment Community	The Apts at Cambridge Court
Market Area	Baltimore
Date Acquired	8/23/2011
Year Constructed	1999-2002
Number of Units	544
Cost of Acquisition	90,400
Cost of Acquisition Per Unit	166
Somerset Apartments [Member]
Property Acquisitions [Line Items]
Apartment Community	Somerset Apartments
Market Area	Suburban D.C.
Date Acquired	10/11/2011
Year Constructed	2006
Number of Units	108
Cost of Acquisition	20,250
Cost of Acquisition Per Unit	188
Newport Village [Member]
Property Acquisitions [Line Items]
Apartment Community	Newport Village
Market Area	Suburban D.C.
Date Acquired	10/17/2011
Year Constructed	1969
Number of Units	937
Cost of Acquisition	205,000
Cost of Acquisition Per Unit	219
The Courts At Dulles [Member]
Property Acquisitions [Line Items]
Apartment Community	The Courts at Dulles
Market Area	Suburban D.C.
Date Acquired	11/30/2011
Year Constructed	2000
Number of Units	411
Cost of Acquisition	92,750
Cost of Acquisition Per Unit	226
The Gates Of Deer Grove [Member]
Property Acquisitions [Line Items]
Apartment Community	The Gates of Deer Grove
Market Area	Chicago
Date Acquired	12/15/2011
Year Constructed	1974
Number of Units	204
Cost of Acquisition	$ 20,200
Cost of Acquisition Per Unit	99
Minimum [Member] | Middlebrooke Apartments [Member]
Property Acquisitions [Line Items]
Year Constructed	1974
Minimum [Member] | Westbrooke Apartments [Member]
Property Acquisitions [Line Items]
Year Constructed	1961
Minimum [Member] | Annapolis Roads Apartments [Member]
Property Acquisitions [Line Items]
Year Constructed	1974
Minimum [Member] | The Greens At Columbia [Member]
Property Acquisitions [Line Items]
Year Constructed	1986
Minimum [Member] | Charleston Place [Member]
Property Acquisitions [Line Items]
Year Constructed	1971
Minimum [Member] | The Apts At Cambridge Court [Member]
Property Acquisitions [Line Items]
Year Constructed	1999
Maximum [Member] | Middlebrooke Apartments [Member]
Property Acquisitions [Line Items]
Year Constructed	1977
Maximum [Member] | Westbrooke Apartments [Member]
Property Acquisitions [Line Items]
Year Constructed	1970
Maximum [Member] | Annapolis Roads Apartments [Member]
Property Acquisitions [Line Items]
Year Constructed	1979
Maximum [Member] | The Greens At Columbia [Member]
Property Acquisitions [Line Items]
Year Constructed	1987
Maximum [Member] | Charleston Place [Member]
Property Acquisitions [Line Items]
Year Constructed	1983
Maximum [Member] | The Apts At Cambridge Court [Member]
Property Acquisitions [Line Items]
Year Constructed	2002

[1] This apartment community was fee-managed by the Company prior to acquisition.