Fair Value Measurements (Details 3) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($) Property
Fair value of impaired properties and other assets
Impairment losses	$ 23,289	$ 1,416	$ 5,857
Exit costs included in impairment losses in connection with dispositions	0	0	979
Fair value measurement on a recurring basis [Member]
Fair value of impaired properties and other assets
Assets	5,816	6,156
Fair value measurement on a nonrecurring basis [Member]
Fair value of impaired properties and other assets
Exit costs included in impairment losses in connection with dispositions			419
Fair value measurement on a nonrecurring basis [Member] | Discontinued Operations [Member]
Fair value of impaired properties and other assets
Impairment losses			15,200
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member]
Fair value of impaired properties and other assets
Impairment losses	23,500	$ 1,400
Assets	13,850
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value of impaired properties and other assets
Assets	13,850
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Indicative Bids [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value of impaired properties and other assets
Assets	24,423
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Comparable Sales Price [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value of impaired properties and other assets
Assets	10,850
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Colorado Springs [Member]
Fair value of impaired properties and other assets
Impairment losses	12,800
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Aberdeen, Maryland [Member]
Fair value of impaired properties and other assets
Impairment losses	6,600
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | White Marsh, Maryland [Member]
Fair value of impaired properties and other assets
Impairment losses	2,600
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Land controlled for future development [Member] | Northern Virginia [Member]
Fair value of impaired properties and other assets
Impairment losses	1,300
Assets Held-for-sale, Net [Member] | Fair value measurement on a nonrecurring basis [Member]
Fair value of impaired properties and other assets
Assets	21,423
Assets Held-for-sale, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value of impaired properties and other assets
Assets	$ 21,423
Property Conveyances [Member] | Fair value measurement on a nonrecurring basis [Member]
Fair value of impaired properties and other assets
Impairment losses			11,000
Exit costs included in impairment losses in connection with dispositions			560
Other Properties with Shortened Holding Periods [Member] | Fair value measurement on a nonrecurring basis [Member] | White Marsh, Maryland [Member]
Fair value of impaired properties and other assets
Impairment losses			$ 5,900
Number of properties, shortened holding period | Property			2