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Mortgage Notes Payable, Net (Narrative) (Details) $ in Thousands		12 Months Ended
	Dec. 19, 2019 USD ($) ft²	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Mortgage Notes Payable [Line Items]
Mortgage notes payable, net (amounts related to VIEs of $2,918,806 and $2,929,326 at December 31, 2019 and 2018, respectively)		$ 2,922,408	$ 2,964,572
Number Of Mortgage Loans Accounted For Fair Value		0	0
Repayments of Secured Debt		$ 46,173	$ 18,634	$ 1,317,653
Losses (gains) from early extinguishments of debt		29,540	16,490	(496)
Payment for Debt Extinguishment or Debt Prepayment Cost		$ 28,716	15,973	12,784
Interest Expense Decrease Due To Recording Mortgage Loans At Fair Value				$ 19,600
Minimum [Member]
Mortgage Notes Payable [Line Items]
Number Real Estate Assets Used For Mortgage Loan Collateral		1
Fixed Rate Mortgages [Member]
Mortgage Notes Payable [Line Items]
Mortgage notes payable, net (amounts related to VIEs of $2,918,806 and $2,929,326 at December 31, 2019 and 2018, respectively)		$ 2,900,000	$ 3,000,000
Debt, Weighted Average Interest Rate		3.73%	3.77%
Fixed Rate Mortgages [Member] | Minimum [Member]
Mortgage Notes Payable [Line Items]
Coupon/Stated Rate		3.43%	3.43%
Fixed Rate Mortgages [Member] | Maximum [Member]
Mortgage Notes Payable [Line Items]
Coupon/Stated Rate		6.94%	7.69%
Variable Rate Mortgages [Member]
Mortgage Notes Payable [Line Items]
Number of variable rate mortgage loans		0	0
New Dominion Technology Park, Bldg. One [Member]
Mortgage Notes Payable [Line Items]
Coupon/Stated Rate	7.69%
Repayments of Secured Debt	$ 26,500
Losses (gains) from early extinguishments of debt	1,500
Payment for Debt Extinguishment or Debt Prepayment Cost	$ 1,400
Net Rentable Area (in sf) | ft²	235,000

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