Secured Debt (Details Textual) (USD $)	6 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Secured debt	$ 1,992,401,000	$ 1,908,068,000
Weighted average interest rate	4.22%
Secured Debt (Textuals) [Abstract]
Number of secured credit facilities	5
Secured debt amount which encumbers real estate owned based upon book value	3,100,000,000
Percentage of secured debt which encumbers real estate owned based upon book value	42.20%
Secured debt amount of real estate owned which is unencumbered	4,200,000,000
Percentage of secured debt of real estate owned which is unencumbered	57.80%
Credit facilities with aggregate commitment	1,400,000,000
Initial term of credit facilities	10 years
Unamortized fair market adjustment	21,500,000	694,000
Secured debt	1,992,401,000	1,908,068,000
Weighted average interest rate	4.22%
Additional time period for extending facilities	5 years
Mortgage notes payable [Member] \| Fixed Rate Debt [Member]
Debt Instrument [Line Items]
Secured debt	564,129,000
Weighted average interest rate	4.95%
Notes payable minimum interest rates range	1.93%
Notes payable maximum interest rates range	6.60%
Secured Debt (Textuals) [Abstract]
Secured debt	564,129,000
Weighted average interest rate	4.95%
Mortgage notes payable [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Secured debt	177,201,000
Weighted average interest rate	1.94%
Notes payable minimum interest rates range	0.93%
Notes payable maximum interest rates range	3.45%
Secured Debt (Textuals) [Abstract]
Secured debt	177,201,000
Weighted average interest rate	1.94%
Tax Exempt Notes Payable [Member] \| Fixed Rate Debt [Member]
Debt Instrument [Line Items]
Secured debt	0
Secured Debt (Textuals) [Abstract]
Secured debt	0
Tax Exempt Notes Payable [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Secured debt	94,700,000
Weighted average interest rate	0.83%
Secured Debt (Textuals) [Abstract]
Secured debt	94,700,000
Weighted average interest rate	0.83%
Tax - Exempt Secured Notes Payable [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Notes payable minimum interest rates range	0.77%
Notes payable maximum interest rates range	0.86%
Fixed Rate Debt [Member]
Debt Instrument [Line Items]
Secured debt	1,460,049,000
Weighted average interest rate	5.18%
Secured Debt (Textuals) [Abstract]
Secured debt	1,460,049,000
Weighted average interest rate	5.18%
Variable Rate Debt [Member]
Debt Instrument [Line Items]
Secured debt	532,352,000,000
Weighted average interest rate	1.58%
Secured Debt (Textuals) [Abstract]
Secured debt	$ 532,352,000,000
Weighted average interest rate	1.58%
Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date range, start	August 2011
Debt instrument, maturity date range, end	42461
Tax Exempt Secured Note Payable [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date range, start	August 2019
Debt instrument, maturity date range, end	47543

Additional time period for extending facilities.

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- Definition

Initial term of credit facilities.

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- Definition

Number of secured credit facilities.

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- Definition

Secured Debt, Encumbers Real Estate Owned, Amount.

+ Details

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- Definition

Secured Debt, Encumbers Real Estate Owned, Percent.

+ Details

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- Definition

Secured Debt.

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- Definition

Secured Debt, Unencumbered Real Estate Owned, Amount.

+ Details

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- Definition

Secured Debt, Unencumbered Real Estate Owned, Percent.

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- Definition

The average effective interest rate during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 3: 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(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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

+ References

+ Details

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- Definition

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

+ References

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

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).

+ References

Reference 1: 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(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 129
-Paragraph 4
-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.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 129
-Paragraph 2
-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

When presenting a range of maturity dates, the earliest 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).

+ References

+ Details

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name FASB Staff Position (FSP)
-Number APB14-1
-Paragraph 31
-Subparagraph b
-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.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Principles Board Opinion (APB)
-Number 21
-Paragraph 16, 20
-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.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 129
-Paragraph 2, 4
-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.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 19, 22
-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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.

+ References

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 19, 20, 22
-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 16
-Article 9

Reference 4: 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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