v2.4.0.8

Investment in unconsolidated Joint Ventures (Narrative) (Details) (USD $) | 3 Months Ended | 6 Months Ended | 0 Months Ended |  |  | 0 Months Ended |  | 0 Months Ended |  | 0 Months Ended | 6 Months Ended | 0 Months Ended
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Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | May 30, 2013 125 West 55th Street [Member] | Apr. 10, 2013 125 West 55th Street [Member] sqft | Feb. 28, 2013 Annapolis Junction Building Six [Member] sqft | May 31, 2013 500 North Capitol [Member] | Jun. 30, 2013 500 North Capitol [Member] sqft | Apr. 04, 2013 Annapolis Junction Building Seven [Member] Y | Apr. 10, 2013 Mountain View Research [Member] | Apr. 10, 2013 Mountain View Technology [Member] | Apr. 10, 2013 Mountain View Research And Technology Parks [Member] | Jun. 05, 2013 540 Madison Avenue [Member] | Apr. 10, 2013 Company [Member] | May 30, 2013 Company [Member] 125 West 55th Street [Member] | Apr. 10, 2013 Mountain View Research And Technology Parks [Member] | May 31, 2013 767 Fifth Avenue (The GM Building) [Member] partners sqft | Jun. 30, 2012 767 Fifth Avenue (The GM Building) [Member] | Apr. 10, 2013 Mountain View Technology [Member] | Apr. 10, 2013 Mountain View Research [Member] | May 31, 2013 Construction Loans [Member] 500 North Capitol [Member] | May 31, 2013 Construction Loans [Member] Company [Member] 500 North Capitol [Member] | May 31, 2013 First Mortgage [Member] 500 North Capitol [Member] | Jun. 05, 2013 First Mortgage [Member] 540 Madison Avenue [Member] | Jun. 05, 2013 Prior to refinance first mortgage loan [Member] 540 Madison Avenue [Member] | Jun. 05, 2013 Prior to refinance first mortgage loan [Member] Company [Member] 540 Madison Avenue [Member]
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Joint Venture Partners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
Contractual Sales Price |  |  |  |  |  | $ 470,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying Value of Debt Assumed in Sale |  |  |  |  | 198,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of Notes Payable |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,700,000 | 8,600,000 |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  |  |  |  |  |  | 1.65% | 2.00% | 2.50% |  |  |  |  |  |  |  |  |  | 1.65% |  |  | 1.50% |  |
Maturity date |  |  |  |  | Mar. 10, 2020 |  |  |  |  | Apr. 04, 2016 | May 31, 2014 | Nov. 22, 2014 |  |  |  |  |  |  |  |  |  | Oct. 14, 2014 |  | Jun. 06, 2023 | Jun. 05, 2018 | Jul. 11, 2013 |
Proceeds from Sale of Equity Method Investments |  |  | 201,182,000 | 3,057,000 | 253,700,000 |  |  |  |  |  |  |  |  |  |  | 152,200,000 |  |  |  |  |  |  |  |  |  |  |
Losses from early extinguishment of debt | (152,000) | (274,000) | (152,000) | (1,041,000) |  |  |  |  |  |  |  |  | 400,000 |  | 200,000 |  |  |  |  |  |  | 1,000,000 | 300,000 |  |  | 300,000 | 200,000
Proceeds from Issuance of Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 105,000,000 | 120,000,000 |  |
Coupon/Stated Rate |  |  |  |  | 6.09% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.15% |  | 5.20% |
Debt Instrument, Description of Variable Rate Basis |  |  |  |  |  |  |  | LIBOR plus 1.65% |  |  | LIBOR plus 2.00% | LIBOR plus 2.50% |  | LIBOR plus 1.50% |  |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investment, Ownership Percentage |  |  |  |  |  | 60.00% | 50.00% | 30.00% | 30.00% | 50.00% |  |  |  | 60.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Ownerhip upon acquistion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |
Gain on consolidation of joint ventures | 387,801,000 | 0 | 387,801,000 | 0 |  |  |  |  |  |  |  |  | 24,400,000 |  |  |  |  | 363,400,000 | 363,400,000 |  |  |  |  |  |  |  |
Construction Loan Facility Borrowing Capacity |  |  |  |  |  |  |  |  |  | 22,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area |  |  |  |  |  | 588,000 | 119,000 |  | 231,000 |  |  |  |  |  |  |  |  | 1,800,000 |  |  |  |  |  |  |  |  |
Gain on sale or real estate |  |  |  |  | 43,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of extensions |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of Secured Debt |  |  | $ 79,865,000 | $ 219,865,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,000,000 | $ 90,000,000 | $ 90,600,000 |  |  |  | $ 118,000,000 |
Extension Option (in years) |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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