Real Estate Properties - Summary of Properties Held for Sale (Details) $ in Thousands	Sep. 30, 2015 USD ($) ft² property building	Dec. 31, 2014 property	Sep. 30, 2014 ft²
Real Estate Properties [Line Items]
Number of Properties | property		0
Square Footage (in sqft)	25,286,000		42,919,000
Held-for-sale
Real Estate Properties [Line Items]
Number of Properties | property	7
Number of Buildings | building	13
Square Footage (in sqft)	1,263,809
Gross Sales Price | $	$ 131,150
Held-for-sale | Georgia Portfolio
Real Estate Properties [Line Items]
Number of Properties | property	4
Number of Buildings | building	5
Square Footage (in sqft)	642,583
Gross Sales Price | $	$ 48,550
Held-for-sale | CBD | One South Church Avenue
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft)	240,811
Gross Sales Price | $	$ 32,000
Held-for-sale | CBD | 775 Ridge Lake Boulevard
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft)	120,678
Gross Sales Price | $	$ 16,300
Held-for-sale | CBD | One Park Square
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	6
Square Footage (in sqft)	259,737
Gross Sales Price | $	$ 34,300
Held-for-sale | Suburban property | 9040 Roswell Road
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft)	178,941
Held-for-sale | Suburban property | The Exchange
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	2
Square Footage (in sqft)	187,632
Held-for-sale | Suburban property | 3920 Arkwright Road
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft)	196,156
Held-for-sale | Suburban property | 1775 West Oak Commons Court
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft)	79,854