Net Income Per Common Share (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Basic:
Income from continuing operations available to common stockholders	$ 24,946,000		$ 37,078,000		$ 50,149,000		$ 50,379,000
Income from continuing operations available to common stockholders (in shares)	37,292,720		34,570,772		37,149,120		34,299,331
Income from continuing operations available to common stockholders (in dollars per share)	$ 0.67		$ 1.07		$ 1.35		$ 1.47
Income from discontinued operations					0		9,421,000
Income from discontinued operations (in shares)					37,149		34,299,000
Income from discontinued operations (in dollars per share)	$ 0		$ 0		$ 0		$ 0.27
Income available to common stockholders	24,946,000		37,078,000		50,149,000		59,800,000
Income available to common stockholders (in dollars per share)	$ 0.67		$ 1.07		$ 1.35		$ 1.74
Effect of Dilutive Securities	0	[1]	54,000	[1]	0	[1]	108,000	[1]
Effect of Dilutive Securities (in shares)	98,000	[1]	137,000	[1]	93	[1]	132,000	[1]
Diluted:
Income from continuing operations available to common stockholders	25,203,000		37,132,000		50,149,000		50,487,000
Adjusted income from continuing operations available to common stockholders (in shares)	37,092,000		34,708,000		37,242		34,431,000
Adjusted income from continuing operations available to common stockholders (in dollars per share)	$ 0.67		$ 1.07		$ 1.35		$ 1.47
Adjusted income from discontinued operations available to common stockholders					0		9,421,000
Adjusted income from discontinued operations available to common stockholders (in shares)					37,242		34,431,000
Adjusted income from discontinued operations available to common stockholders (in dollars per share)	$ 0		$ 0		$ 0		$ 0.27
Total income (Diluted)					50,149,000		59,908,000
Total income per common share amount (in dollars per share)	$ 0.67		$ 1.07		$ 1.35		$ 1.74
Convertible Limited Partnership Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted convertible limited partnership units (in shares)	2,151,494		2,239,057		2,115,670		2,242,112
Net Income (Loss) Allocated to Limited Partners	$ 1,500,000		$ 2,500,000		$ 3,000,000		$ 4,100,000
Anti-dilutive securities (in shares)	2,151,494		2,239,057		2,115,670		2,242,112
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted convertible limited partnership units (in shares)	20,825		41,557		20,825		41,557
Anti-dilutive securities (in shares)	20,825		41,557		20,825		41,557

[1] Weighted average convertible limited partnership units of 2,151,494 and 2,239,057, which include vested Series Z-1 incentive units, for the three months ended June 30, 2013, and 2012, respectively, and 2,151,670 and 2,242,112 for the six months ended June 30, 2013 and 2012, respectively, were not included in the determination of diluted EPS because they were anti-dilutive.  Income allocated to convertible limited partnership units, which includes vested Series Z-1 units, aggregating $1.5 million and $2.5 million for the three months ended June 30, 2013 and 2012, respectively, and $3.0 million and $4.1 million for the six months ended June 30, 2013 and 2012, respectively, have been excluded from income available to common stock holders for the calculation of diluted income per common share since these units are excluded from the diluted weighted average common shares for the period as the effect was anti-dilutive.  The Company has the ability to redeem DownREIT limited partnership units for cash and does not consider them to be potentially dilutive securities.