Subsequent Events (Details)				3 Months Ended				9 Months Ended
	Oct. 20, 2017 USD ($)	Oct. 18, 2017 USD ($) item	Oct. 06, 2017 $ / shares	Jun. 30, 2017 $ / shares	Mar. 31, 2017 $ / shares	Jun. 30, 2016 $ / shares	Mar. 31, 2016 $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 $ / shares	Oct. 24, 2017 USD ($)	Oct. 17, 2017 USD ($) item
Subsequent Events
Cash dividend declared per share (in dollars per share) | $ / shares				$ 0.19	$ 0.19	$ 0.19	$ 0.19	$ 0.19	$ 0.19
BAML Revolver
Subsequent Events
Total available								$ 500,000,000
Extension available on debt								12 months
Additional borrowing capacity allowed by exercising an accordion feature								$ 350,000,000
Subsequent Events. | Senior unsecured notes
Subsequent Events
Principal amount of loan										$ 200,000,000
Subsequent Events. | Series A Notes
Subsequent Events
Principal amount of loan										$ 116,000,000
Effective interest rate (as a percent)										3.99%
Subsequent Events. | Series B Notes
Subsequent Events
Principal amount of loan										$ 84,000,000
Effective interest rate (as a percent)										4.26%
Subsequent Events. | Office Property in Baltimore Maryland
Subsequent Events
Net Proceeds	$ 31,600,000
Subsequent Events. | BAML Revolver
Subsequent Events
Total available		$ 600,000,000									$ 500,000,000
Number of optional extensions | item		2
Extension available on debt		6 months
Number of fiscal quarters | item		3									2
Additional borrowing capacity allowed by exercising an accordion feature		$ 500,000,000									$ 350,000,000
Cash distribution declared | Subsequent Events.
Subsequent Events
Cash dividend declared per share (in dollars per share) | $ / shares			$ 0.19