Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2013
Real Estate Fund [Abstract]
Real Estate Fund Accounting [Policy Text Block]	The Fund is accounted for under the AICPA Investment Company Guide and its investments are reported on its balance sheet at fair value, with changes in value each period recognized in earnings. We consolidate the accounts of the Fund into our consolidated financial statements, retaining the fair value basis of accounting.
Fair Value Disclosures [Abstract]
Fair Value Measurement Policy [Policy Text Block]	In determining fair value, we utilize valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible, as well as consider counterparty credit risk in our assessment of fair value. Considerable judgment is necessary to interpret Level 2 and 3 inputs in determining the fair value of our financial and non-financial assets and liabilities. Accordingly, our fair value estimates, which are made at the end of each reporting period, may be different than the amounts that may ultimately be realized upon sale or disposition of these assets. Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis Financial assets and liabilities that are measured at fair value in our consolidated financial statements consist of (i) marketable securities, (ii) Real Estate Fund investments, (iii) the assets in our deferred compensation plan (for which there is a corresponding liability on our consolidated balance sheet), (iv) interest rate swaps and (v) mandatorily redeemable instruments (Series G-1 through G-4 convertible preferred units and Series D-13 cumulative redeemable preferred units). Fair Value Measurements on a Nonrecurring Basis Assets measured at fair value on a nonrecurring basis on our consolidated balance sheets consist primarily of our investment in Toys “R” Us and real estate assets that were written-down to estimated fair value at December 31, 2012. The fair values of these assets were determined using widely accepted valuation techniques, including (i) discounted cash flow analysis, which considers, among other things, leasing assumptions, growth rates, discount rates and terminal capitalization rates, (ii) income capitalization approach, which considers prevailing market capitalization rates, and (iii) comparable sales activity. Generally, we consider multiple valuation techniques when measuring fair values but in certain circumstances, a single valuation technique may be appropriate. Financial Assets and Liabilities not Measured at Fair Value Financial assets and liabilities that are not measured at fair value on our consolidated balance sheets include cash equivalents (primarily money market funds, which invest in obligations of the United States government), mortgage and mezzanine loans receivable and our secured and unsecured debt. Estimates of the fair value of these instruments are determined by the standard practice of modeling the contractual cash flows required under the instrument and discounting them back to their present value at the appropriate current risk adjusted interest rate, which is provided by a third-party specialist. For floating rate debt, we use forward rates derived from observable market yield curves to project the expected cash flows we would be required to make under the instrument. The fair value of cash equivalents and borrowings under our revolving credit facility is classified as Level 1, and the fair value of our mortgage and mezzanine loans receivable is classified as Level 3. The fair value of our secured and unsecured debt are classified as Level 2.

Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2013

Real Estate Fund Accounting [Policy Text Block]

The Fund is accounted for under the AICPA Investment Company Guide and its investments are reported on its balance sheet at fair value, with changes in value each period recognized in earnings. We consolidate the accounts of the Fund into our consolidated financial statements, retaining the fair value basis of accounting.

Fair Value Disclosures [Abstract]

Fair Value Measurement Policy [Policy Text Block]

In determining fair value, we utilize valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible, as well as consider counterparty credit risk in our assessment of fair value. Considerable judgment is necessary to interpret Level 2 and 3 inputs in determining the fair value of our financial and non-financial assets and liabilities. Accordingly, our fair value estimates, which are made at the end of each reporting period, may be different than the amounts that may ultimately be realized upon sale or disposition of these assets.

Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis

Financial assets and liabilities that are measured at fair value in our consolidated financial statements consist of (i) marketable securities, (ii) Real Estate Fund investments, (iii) the assets in our deferred compensation plan (for which there is a corresponding liability on our consolidated balance sheet), (iv) interest rate swaps and (v) mandatorily redeemable instruments (Series G-1 through G-4 convertible preferred units and Series D-13 cumulative redeemable preferred units).

Fair Value Measurements on a Nonrecurring Basis

Assets measured at fair value on a nonrecurring basis on our consolidated balance sheets consist primarily of our investment in Toys “R” Us and real estate assets that were written-down to estimated fair value at December 31, 2012. The fair values of these assets were determined using widely accepted valuation techniques, including (i) discounted cash flow analysis, which considers, among other things, leasing assumptions, growth rates, discount rates and terminal capitalization rates, (ii) income capitalization approach, which considers prevailing market capitalization rates, and (iii) comparable sales activity. Generally, we consider multiple valuation techniques when measuring fair values but in certain circumstances, a single valuation technique may be appropriate.

Financial Assets and Liabilities not Measured at Fair Value

Financial assets and liabilities that are not measured at fair value on our consolidated balance sheets include cash equivalents (primarily money market funds, which invest in obligations of the United States government), mortgage and mezzanine loans receivable and our secured and unsecured debt. Estimates of the fair value of these instruments are determined by the standard practice of modeling the contractual cash flows required under the instrument and discounting them back to their present value at the appropriate current risk adjusted interest rate, which is provided by a third-party specialist. For floating rate debt, we use forward rates derived from observable market yield curves to project the expected cash flows we would be required to make under the instrument. The fair value of cash equivalents and borrowings under our revolving credit facility is classified as Level 1, and the fair value of our mortgage and mezzanine loans receivable is classified as Level 3. The fair value of our secured and unsecured debt are classified as Level 2.