Stockholders' Equity (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended			12 Months Ended
	Feb. 04, 2012	Sep. 23, 2011	Sep. 20, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Sep. 17, 2010	Dec. 03, 2009
Class of Stock [Line Items]
Common stock shares repurchased				2,291,160
Shares repurchases				0	0	0
Common stock, shares authorized to sell							3,600,000	3,700,000
Shares issued					0
Gross proceeds, maximum								$ 150,000
Common stock shares for Public Equity Offering Program			6,000,000
Price per share for Public Equity Offering Program			$ 58.50
Aggregate net proceeds		336,800
Purchase of common stock offered		6,000,000
Maximum monthly investment in DRIP				$ 10
Remaining shares available under ATM program				395,893
Common stock dividends declared per share	$ 0.66			$ 2.48	$ 2.32	$ 2.68
Common stock, shares outstanding				48,321,305	37,949,229
Umbrella partnership real estate investment trust, shares outstanding				10,739,773
Total common share equivalents				59,061,078
Stock Options [Member]
Class of Stock [Line Items]
Potential Dilutive shares issuable				172,810	1,702,783	2,828,018
Senior Notes [Member]
Class of Stock [Line Items]
Potential Dilutive shares issuable					331,257	331,257