Real Estate Investments - Dispositions (Textual) (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2017 USD ($) a property Parcel	Mar. 31, 2017 USD ($) segment	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) a property Parcel	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) a property
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain (loss) on disposition of real estate	$ 1,088,000		$ (727,000)	$ 8,411,000	$ 114,729,000
Assets held for sale, net	$ 5,569,000			$ 5,569,000		$ 41,718,000
Number of Properties | property	99			99
Impairment charges	$ 327,000		$ 5,679,000	$ 3,057,000	$ 13,069,000
Aggregate carrying values of properties	$ 3,769,678,000			3,769,678,000		3,586,295,000
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain (loss) on disposition of real estate				$ 0
Acreage of land | a	14.9			14.9
Assets Held-for-sale [Member] | Operating Real Estate Investments, Net [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, net						40,700,000
Assets Held-for-sale [Member] | Land Held For Development [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held for sale, net						$ 1,000,000
Other Segment [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | segment		4
Carrying value of real estate prior to impairment		$ 10,200,000
Impairment charges				$ 0
Impairment of properties held for use		$ 1,000,000
Impairment Hold Period		10 years
Residual Capitalization Rates		9.00%
Discount Rates		9.25%
Other Segment [Member] | Impairment on Held for Use Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Aggregate carrying values of properties		$ 9,200,000
Other Segment [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Acreage of land | a	50			50
Provision for impairment on assets held for sale				$ 300,000
Number of Parcels | Parcel	1			1
Carrying value of real estate prior to impairment	$ 5,900,000			$ 5,900,000
Aggregate carrying value	$ 5,600,000			$ 5,600,000
Other Segment [Member] | Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Acreage of land | a	50			50		5
Assets held for sale, net	$ 5,600,000			$ 5,600,000
Provision for impairment on assets held for sale				$ 300,000
2970 Market Street (Cira Square) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain (loss) on disposition of real estate		$ 500,000
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property				12
Number of Properties | property	87			87
Office Properties [Member] | Other Segment [Member] | Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property						2
Office Properties [Member] | Metropolitan DC [Member] | Assets Held-for-sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property						3
Retail Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property				1
Mixed-use Property [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property				1