Leases - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Property State	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Property State	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Leases [Line Items]
Number of properties in portfolio | Property	873		873
Number of states in which our properties are located | State	27		27
Revenues from rental properties included in continuing operations	$ 24,913,000	$ 23,989,000	$ 73,175,000	$ 71,936,000
Revenues from rental properties	24,559,000	23,192,000	71,876,000	69,427,000
Rental revenue increase due to revenue recognition adjustments included in continuing operations	354,000	797,000	1,299,000	2,509,000
Deferred rent receivables reserves	0	0	0	0
Real Estate Taxes and other municipal charges paid then reimbursed by tenants included in revenues and expenses from continuing operations	3,799,000	3,736,000	10,716,000	10,846,000
Lease origination costs	42,000	$ 148,000	42,000	$ 148,000
Net investment in direct financing leases	90,261,000		90,261,000		$ 92,097,000
Net Investments in direct financing lease, minimum lease payments receivable	157,623,000		157,623,000		167,064,000
Net Investment in direct financing lease, unguaranteed estimated residual value	13,979,000		13,979,000		13,979,000
Net Investment in direct financing lease, deferred income	$ 81,341,000		$ 81,341,000		$ 88,946,000
Third Parties [Member]
Leases [Line Items]
Number of properties leased | Property	81		81
Owned Properties [Member]
Leases [Line Items]
Number of properties | Property	792		792