Real Estate Investments - Dispositions (Textual) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) property segment	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain on disposition of real estate	$ 7,323	$ 115,456
Number of Properties | property	104
Aggregate carrying values of properties	$ 3,702,204		$ 3,586,295
Other Segment [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | segment	4
Impairment of properties held for use	$ 1,000
Carrying value of real estate prior to impairment	$ 10,200
Impairment Hold Period	10 years
Residual Capitalization Rates	9.00%
Discount Rates	9.25%
Impairment on Held for Use Properties [Member] | Other Segment [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Aggregate carrying values of properties	$ 9,200
2970 Market Street (Cira Square) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net gain on disposition of real estate	$ 500
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property	8
Number of Properties | property	90
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