Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Book value	$ 4,446,170	$ 3,809,589
Fair value	4,891,881	4,117,230
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues	999,162
Payoffs	(360,426)
Adjustments to fair value	135,915
Fixed Rate Secured Debt [Member]
Book value	1,149,541	1,167,188
Fair value	1,251,477	1,256,331
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues	100,826
Payoffs	(116,319)
Adjustments to fair value	10,639
Variable Rate Secured Debt [Member]
Book value	18,412	6,045
Fair value	18,386	6,045
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues	13,336
Payoffs	(968)
Adjustments to fair value	(27)
Unsecured Debt [Member]
Book value	2,993,217	2,616,063
Fair value	3,336,386	2,834,610
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues	600,000
Payoffs	(222,846)
Adjustments to fair value	124,622
Unsecured Lines Of Credit [Member]
Book value	285,000	20,293
Fair value	285,632	20,244
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues	285,000
Payoffs	(20,293)
Adjustments to fair value	$ 681

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.

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- Definition

Amount of 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, issues, 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 Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: 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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- Definition

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.

+ References

+ Details

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- Definition

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

+ Details

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 7

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20, 22

-Article 5

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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