Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Debt and Capital Lease Obligations	$ 3,809,589	$ 4,207,079
Secured Debt	1,173,233	1,065,628
Book value	3,809,589	4,207,079
Fair value	4,117,230	4,450,343
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues	180,755
Payoffs	(579,447)
Adjustments to fair value	65,579
Long-term Line of Credit	20,293	193,046
Unsecured Debt	2,616,063	2,948,405
Fixed Rate Secured Debt [Member]
Secured Debt	1,167,188	1,042,722
Book value	1,167,188	1,042,722
Fair value	1,256,331	1,069,562
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues	178,507
Payoffs	(53,154)
Adjustments to fair value	61,416
Debt, Weighted Average Interest Rate	6.25%	6.41%
Variable Rate Secured Debt [Member]
Secured Debt	6,045	22,906
Book value	6,045	22,906
Fair value	6,045	22,906
Payoffs	(16,861)
Adjustments to fair value	0
Debt, Weighted Average Interest Rate	0.21%	3.69%
Fixed Rate Unsecured Debt [Member]
Debt, Weighted Average Interest Rate	6.56%	6.43%
Unsecured Debt	2,616,063	2,948,405
Unsecured Debt [Member]
Book value	2,616,063	2,948,405
Fair value	2,834,610	3,164,651
Payoffs	(334,432)
Adjustments to fair value	4,391
Unsecured Lines Of Credit [Member]
Book value	20,293	193,046
Fair value	20,244	193,224
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues	2,248
Payoffs	(175,000)
Adjustments to fair value	$ (228)
Debt, Weighted Average Interest Rate	1.14%	2.83%

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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Reflects the calculation as of the balance sheet date of the average interest rate weighted by the amount of debt outstanding by type or by instrument at that time.

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No definition available.

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Issues that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

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The increase (decrease) of liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period. Such increase (decrease) may be comprised of changes in fair value (gains or losses) whether realized or unrealized that have been included in earnings, changed net asset value, or other comprehensive income (a separate component of shareholders' equity); purchases, sales, issuances, or settlements of assets; and net transfers in to or out of this category within the fair value hierarchy.

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-Paragraph 32

-Subparagraph c(1), c(2), c(3)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Settlements that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

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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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Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date. May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.

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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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