Future Minimum Lease Payments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Siemens Real Estate [Member]
Future Minimum Leases Payments Under Leases [Line Items]
Total contribution of base commercial rental	36.20%	37.10%
AT&T [Member]
Future Minimum Leases Payments Under Leases [Line Items]
Total contribution of base commercial rental	28.30%	14.80%
Westinghouse Electric [Member]
Future Minimum Leases Payments Under Leases [Line Items]
Total contribution of base commercial rental	21.70%
Ryerson Inc. [Member]
Future Minimum Leases Payments Under Leases [Line Items]
Total contribution of base commercial rental	12.90%