Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Dec. 31, 2011
Information regarding indebtedness
Senior Unsecured Debt, Net	$ 640,182	$ 640,227
Unamortized Discounts	4,240	4,625
Senior Unsecured Notes, Gross	644,422	644,852
Unsecured Credit Facility	115,000	149,000
Mortgage and Other Loans
Mortgage and Other Loans Payable, Net	687,139	690,256
Unamortized Premiums	(289)	(305)
Mortgage and Other Loans Payable, Gross	686,850	689,951
Secured Debt [Member]
Information regarding indebtedness
Interest Rate, Maximum	9.25%
Interest Rate, Minimum	4.45%
Effective Interest Rate, Maximum	9.25%
Effective Interest Rate, Minimum	4.45%
Maturity Date Range, End	10/1/2021'
Maturity Date Range, Start	1/1/2013'
Unsecured Debt [Member]
Information regarding indebtedness
Interest Rate	2.342%
Effective Interest Rate	2.342%
Maturity Date	Dec. 12, 2014
2016 Notes [Member]
Information regarding indebtedness
Senior Unsecured Debt, Net	159,469	159,455
Interest Rate	5.75%
Effective Interest Rate	5.91%
Maturity Date	Jan. 15, 2016
2017 Notes [Member]
Information regarding indebtedness
Senior Unsecured Debt, Net	59,602	59,600
Interest Rate	7.50%
Effective Interest Rate	7.52%
Maturity Date	Dec. 01, 2017
2027 Notes [Member]
Information regarding indebtedness
Senior Unsecured Debt, Net	6,065	6,065
Interest Rate	7.15%
Effective Interest Rate	7.11%
Maturity Date	May 15, 2027
2028 Notes [Member]
Information regarding indebtedness
Senior Unsecured Debt, Net	124,465	124,894
Interest Rate	7.60%
Effective Interest Rate	8.13%
Maturity Date	Jul. 15, 2028
2012 Notes [Member]
Information regarding indebtedness
Senior Unsecured Debt, Net	61,827	61,817
Interest Rate	6.875%
Effective Interest Rate	6.85%
Maturity Date	Apr. 15, 2012
2032 Notes [Member]
Information regarding indebtedness
Senior Unsecured Debt, Net	34,687	34,683
Interest Rate	7.75%
Effective Interest Rate	7.87%
Maturity Date	Apr. 15, 2032
2014 Notes [Member]
Information regarding indebtedness
Senior Unsecured Debt, Net	87,344	86,997
Interest Rate	6.42%
Effective Interest Rate	6.54%
Maturity Date	Jun. 01, 2014
2017 II Notes [Member]
Information regarding indebtedness
Senior Unsecured Debt, Net	$ 106,723	$ 106,716
Interest Rate	5.95%
Effective Interest Rate	6.37%
Maturity Date	May 15, 2017

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Unsecured debt gross.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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When presenting a range of interest rates, the highest effective rate for funds borrowed under the debt agreement as of the balance sheet date.

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Interest rate stated in the contractual debt agreement.

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When presenting a range of interest rates, the highest stated (contractual) rate for funds borrowed under the debt agreement as of the balance sheet date.

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Date when the debt instrument is scheduled to be fully repaid, which may be presented in a variety of ways (year, month and year, day, month and year, quarter, etc.).

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When presenting a range of maturity dates, the latest date when the outstanding debt instruments are required to be repaid. May be presented in a variety of ways (for example: year only, month and year, day, month and year, and number of years).

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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The effective interest rate at the end of the reporting period.

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Carrying value as of the balance sheet date , including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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