Subsequent Events subsequent Events (Details) $ in Thousands		1 Months Ended	9 Months Ended
	Oct. 01, 2015 USD ($) a	Nov. 06, 2015 USD ($) swaps yr	Sep. 30, 2015 USD ($) swaps yr	Sep. 30, 2014 USD ($)	Oct. 07, 2015 USD ($) ft² a Real_Estate_Properties Vehicles	Feb. 19, 2015 USD ($) a
Subsequent Event [Line Items]
Repayments of Secured Debt			$ 20,137	$ 82,030
Interest Rate Swap [Member]
Subsequent Event [Line Items]
Notional Amount			$ 775,000
767 Fifth Partners LLC [Member]
Subsequent Event [Line Items]
Ownership Percentage by the Company			60.00%
767 Fifth Partners LLC [Member] | Interest Rate Swap [Member]
Subsequent Event [Line Items]
Number of interest rate swap agreements entered into (swap contracts) | swaps			9
Maximum period of hedging exposure to the variability in future cash flows for forecasted transactions (in years) | yr			10
Average Fixed Interest Rate			2.801%
Notional Amount			$ 250,000
767 Fifth Partners LLC [Member] | Interest Rate Swap [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of interest rate swap agreements entered into (swap contracts) | swaps		1
Maximum period of hedging exposure to the variability in future cash flows for forecasted transactions (in years) | yr		10
Average Fixed Interest Rate		2.379%
Notional Amount		$ 25,000
Operating Partnership [Member] | Interest Rate Swap [Member]
Subsequent Event [Line Items]
Number of interest rate swap agreements entered into (swap contracts) | swaps			16
Maximum period of hedging exposure to the variability in future cash flows for forecasted transactions (in years) | yr			10
Average Fixed Interest Rate			2.437%
Notional Amount			$ 525,000
Operating Partnership [Member] | Interest Rate Swap [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of interest rate swap agreements entered into (swap contracts) | swaps		1
Maximum period of hedging exposure to the variability in future cash flows for forecasted transactions (in years) | yr		10
Average Fixed Interest Rate		2.129%
Notional Amount		$ 25,000
Washingtonian North land parcel [Member]
Subsequent Event [Line Items]
Sale Price Of Sold Property						$ 8,700
Land sold (in acres) | a						8.5
Area of Land | a						27
Washingtonian North land parcel [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Sale Price Of Sold Property	$ 13,300
Land sold (in acres) | a	5.8
Area of Land | a	19.3
Innovation Place [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Vehicles In Structured Parking | Vehicles					1,699
Net Rentable Area (in sf) | ft²					574,000
Sale Price Of Sold Property					$ 207,000
Area of Land | a					26
Secured Debt [Member] | Kingstowne Two and Kingstowne Retail [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Repayments of Secured Debt	$ 29,800
Coupon/Stated Rate	5.99%
Occupied [Member] | Innovation Place [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of real estate properties | Real_Estate_Properties					1
Vacant [Member] | Innovation Place [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Net Rentable Area (in sf) | ft²					463,000
Number of real estate properties | Real_Estate_Properties					3
Parking structures [Member] | Innovation Place [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of real estate properties | Real_Estate_Properties					2
Future development [Member] | Innovation Place [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Vehicles In Structured Parking | Vehicles					3,000
Net Rentable Area (in sf) | ft²					537,000