Leases - Additional Information (Detail)	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) Property State	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Property State	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Leases [Line Items]
Number of properties in portfolio | Property	907								907
Number of states in which our properties are located | State	28								28
Revenues from rental properties included in continuing operations	$ 28,158,000	$ 24,913,000	$ 24,364,000	$ 23,897,000	$ 24,775,000	$ 23,989,000	$ 23,874,000	$ 24,073,000	$ 101,332,000	$ 96,711,000	$ 91,822,000
Revenues from rental properties									99,355,000	93,294,000	87,280,000
Rental revenue increase due to revenue recognition adjustments included in continuing operations									1,976,000	3,417,000	4,471,000
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									15,829,000	15,017,000	15,256,000
Lease origination costs	126,000				148,000				126,000	148,000	120,000
Net investment in direct financing leases	89,587,000				92,097,000				89,587,000	92,097,000
Net Investments in direct financing lease, minimum lease payments receivable	154,441,000				167,064,000				154,441,000	167,064,000
Net Investment in direct financing lease, unguaranteed estimated residual value	13,979,000				13,979,000				13,979,000	13,979,000
Net Investment in direct financing lease, deferred income	78,833,000				$ 88,946,000				$ 78,833,000	88,946,000
Remaining lease term									9 years
2018	6,062,000								$ 6,062,000
2019	5,378,000								5,378,000
2020	4,271,000								4,271,000
2021	3,358,000								3,358,000
2022	1,753,000								1,753,000
Thereafter	$ 3,328,000								3,328,000
Rent expense on operating lease									5,091,000	5,376,000	5,918,000
Rent received under sublease									$ 9,296,000	$ 9,153,000	$ 9,653,000
Third Parties [Member]
Leases [Line Items]
Number of properties leased | Property	79								79
Owned Properties [Member]
Leases [Line Items]
Number of properties | Property	828								828