Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Basic:
Income from continuing operations available to common stockholders									$ 110,373		$ 32,256		$ 32,251
Income from continuing operations available to common stockholders (in shares)									35,032,491		32,541,792		29,667,064
Income from continuing operations available to common stockholders (in dollars per share)									$ 3.15		$ 0.99		$ 1.09
Income from discontinued operations									9,439		8,112		1,513
Income from discontinued operations (in shares)									35,032,491		32,541,792		29,667,064
Income from discontinued operations (in dollars per share)									$ 0.27		$ 0.25		$ 0.05
Income available to common stockholders	43,793	16,219	37,078	22,722	13,937	7,687	10,326	8,418	119,812		40,368		33,764
Income available to common stockholders (in dollars per share)	$ 1.22	$ 0.46	$ 1.07	$ 0.67	$ 0.42	$ 0.23	$ 0.32	$ 0.27	$ 3.42		$ 1.24		$ 1.14
Effect of Dilutive Securities									0	[1]	0	[1]	0	[1]
Effect of Dilutive Securities (in shares)									92,430	[1]	86,922	[1]	67,319	[1]
Diluted:
Income from continuing operations available to common stockholders									110,373	[1]	32,256	[1]	32,251	[1]
Adjusted income from continuing operations available to common stockholders ( 1 ) (in shares)									35,124,921	[1]	32,628,714	[1]	29,734,383	[1]
Adjusted income from continuing operations available to common stockholders ( 1 ) (in dollars per share)									$ 3.14	[1]	$ 0.99	[1]	$ 1.09	[1]
Adjusted income from discontinued operations available to common stockholders									9,439		8,112		1,513
Adjusted income from discontinued operations available to common stockholders (in shares)									35,124,921		32,628,714		29,734,383
Adjusted income from discontinued operations available to common stockholders (in dollars per share)									$ 0.27		$ 0.25		$ 0.05
Total income (Diluted)									$ 119,812		$ 40,368		$ 33,764
Total income per common share amount (in dollars per share)	$ 1.22	$ 0.45	$ 1.07	$ 0.67	$ 0.42	$ 0.23	$ 0.32	$ 0.27	$ 3.41		$ 1.24		$ 1.14
Convertible Limited Partnership Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted convertible limited partnership units									2,219,046		2,231,807		2,293,886
Anti-dilutive securities (in shares)									2,219,046		2,231,807		2,293,886
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted convertible limited partnership units									263,613		175,500		123,164
Anti-dilutive securities (in shares)									263,613		175,500		123,164

[1] Weighted convertible limited partnership units of 2,219,046, 2,231,807 and 2,293,886, which include vested Series Z incentive units, for the years ended December 31, 2012, 2011 and 2010, respectively, were not included in the determination of diluted EPS because they were 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.