Recent Transactions (Rental Properties Held For Sale) (Narrative) (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) ft² property	Dec. 31, 2015 property
Real Estate Properties [Line Items]
Number Of Properties Sold | property		6
Disposal Group, Disposed Of By Sale, Not Discontinued Operations [Member]
Real Estate Properties [Line Items]
Assets held for sale, Deferred charges and other assets	$ 13,200,000
Assets held for sale, Unbilled rents receivable, net	20,100,000
Assets held for sale, Accounts receivable, net of allowance	1,300,000
Assets held for sale, Accounts payable, accrued expenses and other liabilities	3,400,000
Assets held for sale, Rents received in advance and security deposits	1,200,000
Expected assets to be written off	31,300,000
Expected liabilities to be written off	$ 796,000
Disposal Group, Disposed Of By Sale, Not Discontinued Operations [Member] | 1400L Street, NW in Washington, D.C. and 125 Broad Street in New York, New York [Member]
Real Estate Properties [Line Items]
Area of property (in square feet) | ft²	683,000
Number of properties held for sale | property	2
Estimated expected sales proceeds	$ 272,000,000
Expected disposition completion date	second quarter of 2016