Summary of Significant Accounting Policies - Construction in Progress (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Construction in progress	$ 91,114	$ 63,570
N. Glebe Road
Property, Plant and Equipment [Line Items]
Construction in progress	83,462	58,147
Other
Property, Plant and Equipment [Line Items]
Construction in progress	$ 7,652	$ 5,423