Investment in Real Estate - 2016 Acquisitions - Westwood Portfolio Revenue and Net Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Real Estate Acquisition [Line Items]
Revenues	$ 226,292	$ 223,308	$ 219,469	$ 212,247	$ 209,190	$ 208,749	$ 199,632	$ 194,481	$ 881,316	$ 812,052	$ 742,551
Net income attributable to common stockholders	$ 25,635	$ 30,561	$ 31,684	$ 28,206	$ 29,536	$ 25,614	$ 20,244	$ 19,049	116,086	94,443	85,397
Office rental
Real Estate Acquisition [Line Items]
Revenues									777,931	715,546	645,633
Westwood Portfolio
Real Estate Acquisition [Line Items]
Net income attributable to common stockholders									6,163	6,346	2,998
Net income attributable to common stockholders excluding transaction costs									6,200	6,300	5,000
Westwood Portfolio | Office rental
Real Estate Acquisition [Line Items]
Revenues									$ 102,398	$ 96,106	$ 80,464