Net Income Per Common Share (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Basic [Abstract]
Income from continuing operations available to common stockholders	$ 56,644,000		$ 16,057,000		$ 106,337,000		$ 66,129,000
Income from continuing operations available to common stockholders (in shares)	37,320,562		35,600,772		37,206,895		34,736,311
Income from continuing operations available to common stockholders (in dollars per share)	$ 1.52		$ 0.45		$ 2.86		$ 1.90
Income from discontinued operations available to common stockholders	12,144,000		162,000		12,600,000		9,890,000
Income from discontinued operations available to common stockholders (in shares)	37,321,000		35,601,000		37,207,000		34,736,000
Income from discontinued operations available to common stockholders (in dollars per share)	$ 0.32		$ 0.01		$ 0.34		$ 0.29
Income available to common stockholders	68,788,000		16,219,000		118,937,000		76,019,000
Income available to common stockholders (in dollars per share)	$ 1.84		$ 0.46		$ 3.20		$ 2.19
Effect of dilutive securities	54,000	[1]	0	[1]	0	[1]	0	[1]
Effect of dilutive securities (in shares)	116,000	[1]	99,000	[1]	89,000	[1]	98,000	[1]
Diluted [Abstract]
Income from continuing operations available to common stockholders	56,698,000		16,057,000		106,337,000	[1]	66,129,000	[1]
Adjusted income from continuing operations available to common stockholders (in shares)	37,437,000		35,700,000		37,296,000	[1]	34,834,000	[1]
Adjusted income from continuing operations available to common stockholders (in dollars per share)	$ 1.52		$ 0.44		$ 2.85	[1]	$ 1.90	[1]
Adjusted income from discontinued operations available to common stockholders	12,144,000		162,000		12,600,000		9,890,000
Adjusted income from discontinued operations available to common stockholders (in shares)	37,437,000		35,700,000		37,296,000		34,834,000
Adjusted income from discontinued operations available to common stockholders (in dollars per share)	$ 0.32		$ 0.01		$ 0.34		$ 0.28
Total income (Diluted)	68,842,000		16,219,000		118,937,000		76,019,000
Total income per common share amount (in dollars per share)	$ 1.84		$ 0.45		$ 3.19		$ 2.18
Convertible Limited Partnership Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	2,146,923		2,235,783		2,128,315		2,239,987
Net income (loss) allocated to Limited Partners	$ 4,000,000		$ 1,100,000		$ 7,000,000		$ 5,200,000
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	38,825				38,825

[1] Weighted average convertible limited partnership units of 2,146,923 and 2,235,783, which include vested Series Z-1 incentive units, for the three months ended September 30, 2013, and 2012, respectively, and 2,128,315 and 2,239,987 for the nine months ended September 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 $4.0 million and $1.1 million for the three months ended September 30, 2013 and 2012, respectively, and $7.0 million and $5.2 million for the nine months ended September 30, 2013 and 2012, respectively, have been excluded from income available to common stockholders 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.