Subsequent Events - Narrative (Details) $ / shares in Units, $ in Thousands												12 Months Ended
	Feb. 18, 2020 $ / shares	Feb. 12, 2020 USD ($) ft²	Jan. 10, 2020 $ / shares	Nov. 15, 2019 $ / shares	Oct. 11, 2019 $ / shares	Aug. 15, 2019 $ / shares	Jul. 12, 2019 $ / shares	May 15, 2019 $ / shares	Apr. 11, 2019 $ / shares	Feb. 15, 2019 $ / shares	Jan. 11, 2019 $ / shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) ft²
Disposed of by Sale
Subsequent Event [Line Items]
Property square feet (in sqft) | ft²														6,588,128
Gross sales price | $												$ 812,100	$ 1,031,286	$ 862,600
Series D Preferred Stock
Subsequent Event [Line Items]
Dividend declared (in dollars per share)					$ 0.40625		$ 0.40625		$ 0.40625		$ 0.40625
Dividends paid (in dollars per share)				$ 0.40625		$ 0.40625		$ 0.40625		$ 0.40625
Series D Preferred Stock | Scenario, Forecast
Subsequent Event [Line Items]
Dividends paid (in dollars per share)	$ 0.40625
Subsequent event | 109 Brookline Avenue | Disposed of by Sale
Subsequent Event [Line Items]
Property square feet (in sqft) | ft²		300,000
Gross sales price | $		$ 270,000
Subsequent event | 333 108th Avenue NE | Disposed of by Sale
Subsequent Event [Line Items]
Property square feet (in sqft) | ft²		400,000
Gross sales price | $		$ 401,500
Subsequent event | Series D Preferred Stock
Subsequent Event [Line Items]
Dividend declared (in dollars per share)			$ 0.40625