Real Estate Properties - Schedule of Property Sales (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended			1 Months Ended
	Jan. 31, 2013 building property	Dec. 31, 2014 sqft	Dec. 31, 2013 sqft	Dec. 31, 2012	Dec. 31, 2013 property building sqft	Jun. 30, 2013 property building sqft	Sep. 30, 2012 building property sqft	Jun. 30, 2012 property building sqft	Apr. 30, 2012 property building sqft	Mar. 31, 2013 building property sqft	Oct. 31, 2013 building property sqft	Aug. 31, 2013 property building sqft	May 31, 2013 property building sqft	Apr. 30, 2013 building property sqft	Jun. 30, 2014 property sqft	Nov. 30, 2013 property building sqft	Mar. 31, 2014 sqft
Real Estate Properties [Line Items]
Number of Properties	3
Number of Buildings	18
Square Feet		42,919,000	42,915,000		42,915,000
Gain (Loss) on Sale		$ 0	$ 1,596	$ 0
CBD and suburban properties
Real Estate Properties [Line Items]
Square Feet		5,641,450															5,641,450
Sales of properties resulting in gains and losses
Real Estate Properties [Line Items]
Number of Properties			7	3
Number of Buildings			59	0
Square Feet			2,760,235	298,762	2,760,235
Sales Price			105,207	11,725
Gain (Loss) on Sale			(20,566)	2,039
Sales of properties resulting in gains and losses | Suburban property
Real Estate Properties [Line Items]
Number of Properties					3	1	1	1	1
Number of Buildings					40	1	0	0	0
Square Feet			1,670,104		1,670,104	30,105	208,850	76,978	12,934
Sales Price					89,000	1,600	9,900	1,250	575
Gain (Loss) on Sale					(25,521)	317	1,689	192	158
Sales of properties resulting in gains and losses | Land parcel
Real Estate Properties [Line Items]
Number of Properties						0				0
Number of Buildings						0				0
Square Feet						0				0
Sales Price						2,551				1,806
Gain (Loss) on Sale						1,765				1,596
Sales of properties resulting in gains and losses | Suburban office and industrial properties
Real Estate Properties [Line Items]
Number of Properties	3
Number of Buildings	18
Square Feet	1,060,026
Sales Price	10,250
Gain (Loss) on Sale	1,277
Sales of properties with previously recorded asset impairment losses
Real Estate Properties [Line Items]
Number of Properties			36
Number of Buildings			75
Square Feet			5,583,720		5,583,720
Sales Price			143,925
Gain (Loss) on Sale			0
Sales of properties with previously recorded asset impairment losses | Suburban property
Real Estate Properties [Line Items]
Number of Properties					2	1					9	1	1	1
Number of Buildings					2	2					41	3	1	1
Square Feet			77,394		77,394	356,045					1,211,536	129,452	57,250	618,000
Sales Price					2,020	16,300					43,000	4,100	4,025	830
Gain (Loss) on Sale					0	0					0	0	0	0
Sales of properties with previously recorded asset impairment losses | Suburban property
Real Estate Properties [Line Items]
Number of Properties						1					1
Number of Buildings						1					1
Square Feet						143,802					665,545
Sales Price						5,250					13,900
Gain (Loss) on Sale						0					0
Sales of properties with previously recorded asset impairment losses | Suburban property
Real Estate Properties [Line Items]
Number of Properties											1
Number of Buildings											2
Square Feet											199,418
Sales Price											4,000
Gain (Loss) on Sale											0
Sales of properties with previously recorded asset impairment losses | CBD and suburban properties
Real Estate Properties [Line Items]
Number of Properties															14	18
Number of Buildings															43	21
Square Feet															2,784,098	2,125,278
Sales Price															215,900	50,500
Gain (Loss) on Sale															$ 0	$ 0