Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands	9 Months Ended
Sep. 30, 2011	Dec. 31, 2010
Book value	$ 4,272,323	$ 4,207,079
Fair value	4,530,244	4,450,343
Issuances and assumptions	254,980
Payoffs	(191,186)
Adjustments to fair value	16,107
Fixed Rate Secured Debt [Member]
Book value	1,162,147	1,042,722
Fair value	1,264,303	1,069,562
Issuances and assumptions	143,732
Payoffs	(24,056)
Adjustments to fair value	75,065
Variable Rate Secured Debt [Member]
Book value	22,121	22,906
Fair value	22,121	22,906
Issuances and assumptions	0
Payoffs	(785)
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,783,762	2,948,405
Fair value	2,934,533	3,164,651
Issuances and assumptions	0
Payoffs	(166,345)
Adjustments to fair value	(63,773)
Unsecured Lines Of Credit [Member]
Book value	304,293	193,046
Fair value	309,287	193,224
Issuances and assumptions	111,248
Payoffs	0
Adjustments to fair value	$ 4,815

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

+ References

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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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