Real Estate Properties - Narrative (Details) (USD $)	0 Months Ended	1 Months Ended		3 Months Ended		9 Months Ended
	Jun. 27, 2014	Mar. 31, 2014	Jan. 31, 2013 property	Sep. 30, 2014 property sqft	Sep. 30, 2013 sqft	Sep. 30, 2014 property sqft	Sep. 30, 2013 sqft
Real estate properties information
Real estate improvements						$ 65,900,000	$ 75,400,000
Number of real estate properties held for sale				0		0
Property square feet				42,919,000	42,867,000	42,919,000	42,867,000
Reversal of loss on asset impairment		4,800,000
Number of properties sold			3
Number of buildings sold			18
Impairment loss	2,200,000			122,000	(92,827,000)	(2,238,000)	(101,362,000)
Loss on early extinguishment of debt	3,300,000			0	0	(3,345,000)	0
CBD
Real estate properties information
Number of real estate properties held for use		2
Number of buildings held for use		2
Number of properties sold	1
Number of buildings sold	2
Suburban property
Real estate properties information
Number of real estate properties held for use		29
Number of buildings held for use		65
Number of properties sold	13
Number of buildings sold	41
CBD and suburban properties
Real estate properties information
Property square feet		5,641,450
Property square feet, sold	2,784,098
Aggregate sale price of properties sold, excluding closing costs	$ 215,900,000