Summary of Significant Accounting Policies - Construction in Progress (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Construction in progress	$ 216,545	$ 185,972
Glebe Road
Property, Plant and Equipment [Line Items]
Construction in progress	189,211	162,176
Ashbrook Marketplace
Property, Plant and Equipment [Line Items]
Construction in progress	13,068	11,124
Other
Property, Plant and Equipment [Line Items]
Construction in progress	$ 14,266	$ 12,672