Investment in Office Properties - Summary of Investments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2013 State sqft Property	Mar. 09, 2012 sqft	Dec. 31, 2011 Property
Business Acquisition [Line Items]
Number of real estate properties		43		2
Number of states in which investment properties are located		8
Area of real estate property (in square feet)		17,600,000	337,000
Total Purchase Price	$ 898,400
Atlanta, GA [Member]
Business Acquisition [Line Items]
Total Purchase Price	56,300
Houston, TX [Member]
Business Acquisition [Line Items]
Total Purchase Price	646,700
Jacksonville, FL [Member]
Business Acquisition [Line Items]
Total Purchase Price	130,000
Miami, FL [Member]
Business Acquisition [Line Items]
Total Purchase Price	$ 65,400
Office Building [Member]
Business Acquisition [Line Items]
Number of real estate properties		50