Indebtedness - Activity (Details) (USD $)				12 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 7.25% Senior Notes [Member] Unsecured Debt [Member] Satisfied in Full [Member]	Dec. 31, 2012 6.375% Senior Notes [Member] Unsecured Debt [Member] Y	Dec. 31, 2012 3.375% senior note [Member] Issuance of Debt [Member]	Dec. 31, 2010 4.75% Senior Notes [Member] Issuance of Debt [Member] Y	Dec. 31, 2010 8.5% Senior Notes [Member] Unsecured Debt [Member]	Dec. 31, 2010 7.3% Mortgage Loans [Member] Mortgages [Member]	Dec. 31, 2012 4.125 Senior Notes [Member] Issuance of Debt [Member]	Dec. 31, 2012 Mortgage Loans on Real Estate [Member] Issuance of Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Borrowing Capacity, Amount											$ 45,000,000
Repayments of Senior Debt				246,500,000	230,100,000
Extinguishment of Debt, Amount								169,700,000	119,300,000
Debt Instrument, Interest Rate, Stated Percentage				7.25%	6.375%	3.375%	4.75%	8.50%		4.125%
Maturity Date of Debt Instrument					August 2012	2023		August 2010		2022
Debt, Weighted Average Interest Rate	5.30%	5.80%	6.20%						7.30%		4.84%
Mortgage Loans	302,855,000	290,819,000									34,600,000
Payments of Debt Extinguishment Costs									1,200,000
Write off of Deferred Debt Issuance Cost									936,000
Debt Instrument, Face Amount						$ 300,000,000	$ 350,000,000			$ 400,000,000
Term of Debt					10		10