Schedule III - Real Estate and Accumulated Depreciation (Reconciliation of Real Estate Investments and Accumulated Depreciation) (Parentheticals) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at the Beginning of the Year		$ 4,631,128		$ 4,669,289		$ 4,726,169
Additions:
Acquisitions		182,381		0		347,510
Capital expenditures and assets placed into service		165,941		132,149		109,740
Less:
Dispositions		(442,327)		(126,471)		(474,275)
Retirements		(49,535)		(43,839)		(39,855)
Balance at the end of the year		4,487,588		4,631,128		4,669,289
Assets held for sale		(794,588)	[1]	(27,436)	[1]	0
Operating properties		3,693,000		4,603,692		4,669,289
Tax Basis of Assets, Cost for Income Tax Purposes		3,900,000		3,700,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at the beginning of the year		1,078,996		983,808		954,665
Real Estate Accumulated Depreciation Additions [Abstract]
Depreciation expense — continuing operations		159,080		160,641		150,236
Depreciation expense — discontinued operations		0		0		9,164
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Dispositions		(109,243)		(22,459)		(91,890)
Retirements		(48,217)		(42,994)		(38,367)
Balance at the end of the year		1,080,616		1,078,996		983,808
Asset held for sale		(213,581)		(11,167)		0
Real Estate Investment Property, Accumulated Depreciation		867,035		1,067,829
Aggregate carrying value		40,400
Impairment of properties held for use		27,500
Estimated Fair Value Of Impaired Property		$ 12,900
Equity Method Investment, Ownership Percentage	[2]	0.00%
Tysons Corner [Member] | Secured Debt [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Debt Instrument Maturity Date		Oct. 09, 2015
3141 Fairview Park Drive [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Equity Method Investment, Ownership Percentage		50.00%
Land and Building [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Provision for impairment on assets held for sale		$ 45,400
Aggregate carrying value		426,200
Estimated closing costs		380,800
Accumulated Depreciation, including HFS [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Real Estate Investment Property, Accumulated Depreciation		$ 867,035		$ 1,067,829		$ 983,808

[1] 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.

[2] Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.