Schedule III - Real Estate and Accumulated Depreciation (Reconciliation of Real Estate Investments and Accumulated Depreciation) (Parentheticals) (Details) - USD ($)			12 Months Ended
		Jan. 14, 2016	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at the Beginning of the Year			$ 4,487,588,000		$ 4,631,128,000		$ 4,669,289,000
Additions:
Acquisitions			0		182,381,000		0
Capital expenditures and assets placed into service			213,996,000		165,941,000		132,149,000
Less:
Retirements			(79,022,000)		(49,535,000)		(43,839,000)
Balance at the end of the year			3,659,886,000		4,487,588,000		4,631,128,000
Assets held for sale	[1],[2]		(73,591,000)	[3]	(794,588,000)	[3]	(27,436,000)
Operating properties			3,586,295,000		3,693,000,000		4,603,692,000
Dispositions/impairments/placed into redevelopment			(962,676,000)		(442,327,000)		(126,471,000)
Tax Basis of Assets, Cost for Income Tax Purposes			3,000,000,000		3,900,000,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at the beginning of the year			1,080,616,000		1,078,996,000		983,808,000
Real Estate Accumulated Depreciation Additions [Abstract]
Depreciation expense			131,859,000		159,080,000		160,641,000
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Retirements			(76,973,000)		(48,217,000)		(42,994,000)
Balance at the end of the year			885,392,000		1,080,616,000		1,078,996,000
Assets held for sale			(32,916,000)		(213,581,000)		(11,167,000)
Real Estate Investment Property, Accumulated Depreciation			852,476,000		867,035,000
Dispositions/impairments/placed into redevelopment			(250,110,000)		(109,243,000)		(22,459,000)
Extinguishment of Debt, Amount			0		0		376,174,000
Aggregate carrying value			25,800,000
Impairment of properties held for use			7,300,000
Estimated Fair Value Of Impaired Property			18,500,000
Land and Building [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Provision for impairment on assets held for sale			3,000,000
Aggregate carrying value			13,300,000
Estimated closing costs			10,300,000
Land and Building [Member] | 1200 and 1220 Concord Avenue in Concord [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Provision for impairment on assets held for sale			11,500,000
Aggregate carrying value			43,500,000
Estimated closing costs			32,000,000
Secured Debt [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Extinguishment of Debt, Amount		$ 44,400,000
Repayments of debt		35,500,000
Repayment of prepayment charges		$ 8,900,000
Accumulated Depreciation, including HFS [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Real Estate Investment Property, Accumulated Depreciation			$ 852,476,000		$ 867,035,000		$ 1,067,829,000

[1] As of December 31, 2015, 2970 Market Street was classified as held for sale on the consolidated balance sheets. The property was sold on February 5, 2016. See Note 21, "Subsequent Events," for further information. The sale is not classified as a significant disposition under the accounting guidance for discontinued operations.

[2] As of December 31, 2015, the 58 properties associated with the series of related transactions with Och-Ziff Real Estate were classified as held for sale on the consolidated balance sheets. On February 4, 2016, the Company completed a series of transactions, resulting in the disposition of the properties. See Note 3, “Real Estate Investments,” for further information regarding the disposition. Additionally, as of December 31, 2016, the Company categorized three office properties located in the Metropolitan Washington, D.C. segment and two properties in the Other segment as held for sale in accordance with applicable accounting standards for long lived assets. See Note 3, “Real Estate Investments,” for further information.

[3] Real estate investments related to assets held for sale above represents gross real estate assets and does not include accumulated depreciation or other assets on the balance sheets of the properties held for sale. See Held for Sale section below.