Leases - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Property	Dec. 31, 2016 USD ($) Option	Dec. 31, 2015 USD ($)
Leases Disclosure [Line Items]
Renewal option term	5 years
Number of properties subject to ground leases | Property	14
Amount of expenses relating to applicable leases | $	$ 460,000	$ 450,000	$ 448,000
Minimum
Leases Disclosure [Line Items]
Operating Lease Terms	3 years
Maximum
Leases Disclosure [Line Items]
Operating Lease Terms	15 years
Number of term renewal options | Option		5