Investment in Office Properties - Narrative (Details) $ in Thousands, ft² in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) ft² property state	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) ft² property state	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Real Estate Properties [Line Items]
Number of states in which investment properties are located | state	6								6
Area of real estate property (in acres) | ft²	14.1								14.1
Net income (loss) from properties acquired									$ 67,335	$ 42,943	$ (19,650)
Revenues	$ 112,551	$ 114,843	$ 126,905	$ 119,684	$ 126,381	$ 115,747	$ 110,460	$ 104,113	$ 473,983	456,701	$ 291,579
Office Building
Real Estate Properties [Line Items]
Number of real estate properties | property	35								35
2015 Acquisitions
Real Estate Properties [Line Items]
Net income (loss) from properties acquired									$ 2,600
Revenues									$ 21,000
2014 Acquisitions
Real Estate Properties [Line Items]
Net income (loss) from properties acquired										(2,100)
Revenues										$ 28,900