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Mortgage Notes Payable (Narrative) (Details) $ in Thousands |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 15, 2015 USD ($) | Oct. 01, 2015 USD ($) | Sep. 18, 2015 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Mortgage notes payable |  |  |  | $ 3,438,714 | $ 4,309,484 |
Final maturity date of mortgage notes |  |  |  | Apr. 10, 2022 |  |
Repayments of mortgage notes payable |  |  |  | $ 54,801 | 87,758 | $ 80,311
Investment in Marketable Securities Related to Defeasance |  |  |  | 667,335 | 0 | 0
Mortgage note payable legally defeased |  |  |  | 640,500 | 0 | 0
Gains (losses) from early extinguishments of debt |  |  |  | $ (22,040) | $ (10,633) | 122
Number Of Mortgage Loans Accounted For Fair Value |  |  |  | 2 | 4 |
Outstanding balance of mortgage loans accounted for at fair value |  |  |  | $ 1,500,000 | $ 2,200,000 |
Interest Expense Decrease Due To Recording Mortgage Loans At Fair Value |  |  |  | 55,000 | 52,500 | 34,400
Cumulative Liability Related to the Fair Value of Debt Adjustment |  |  |  | 80,200 | 138,700 |
Financing costs |  |  |  | 61,402 | 69,127 |
Unamortized balance of historical fair value adjustments |  |  |  | 80,235 |  |
Interest expense |  |  |  | $ 432,196 | $ 455,743 | $ 446,880
Minimum [Member] |  |  |  |  |  |
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Number Real Estate Assets Used For Mortgage Loan Collateral |  |  |  | 1 | 1 |
Fixed Rate Mortgages [Member] |  |  |  |  |  |
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Mortgage notes payable |  |  |  | $ 3,400,000 | $ 4,300,000 |
Contractual interest rate range, minimum |  |  |  | 4.75% | 4.75% |
Contractual interest rate range, maximum |  |  |  | 7.69% | 7.69% |
Contractual interest rate, weighted average interest rate |  |  |  | 5.69% | 5.70% |
Variable Rate Mortgages [Member] |  |  |  |  |  |
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Number of variable rate mortgage loans |  |  |  | 0 | 0 |
London Interbank Offered Rate |  |  |  | 0.43% | 0.17% |
Secured Debt [Member] | Kingstowne Two and Kingstowne Retail [Member] |  |  |  |  |  |
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Repayments of mortgage notes payable |  | $ 29,800 |  |  |  |
Interest at a fixed rate |  | 5.99% |  |  |  |
Secured Debt [Member] | Two Hundred Clarendon Street and Garage [Member] |  |  |  |  |  |
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Interest at a fixed rate | 5.68% |  |  |  |  |
Investment in Marketable Securities Related to Defeasance | $ 667,300 |  |  |  |  |
Mortgage note payable legally defeased | 640,500 |  |  |  |  |
Gains (losses) from early extinguishments of debt | (22,000) |  |  |  |  |
Consolidated Properties [Member] | 505 9th Street [Member] |  |  |  |  |  |
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Ownership Percentage by Parent |  |  | 50.00% |  |  |
Mortgage Loan Related to Property Sales |  |  | $ 117,000 |  |  |
Consolidated Properties [Member] | Secured Debt [Member] | 505 9th Street [Member] |  |  |  |  |  |
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Ownership Percentage by Parent |  |  | 50.00% |  |  |
Mortgage Loan Related to Property Sales |  |  | $ 117,000 |  |  |
Interest at a fixed rate |  |  | 5.73% |  |  |
Mortgages [Member] | Secured Debt [Member] | Two Hundred Clarendon Street and Garage [Member] |  |  |  |  |  |
Mortgage Notes Payable [Line Items] |  |  |  |  |  |
Difference between US Treasuries and outstanding principal balance | 26,800 |  |  |  |  |
Financing costs | 1,400 |  |  |  |  |
Unamortized balance of historical fair value adjustments | 4,800 |  |  |  |  |
Interest expense | $ 1,400 |  |  |  |  |

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- Definition For an unclassified balance sheet, the amount as of the balance sheet date of capitalized costs associated with the issuance of debt instruments (for example, legal, accounting, underwriting, printing, and registration costs) that will be charged against earnings over the life of the debt instruments to which such costs pertain. Such amount is before the consideration of accumulated amortization. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28555-108399 + Details
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