Summary of Significant Accounting Policies - Additional Information (Detail)		12 Months Ended
	Aug. 03, 2015 USD ($)	Dec. 31, 2017 USD ($) Tenant Property	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges		$ 9,321,000	$ 12,814,000	$ 17,361,000
Maturity period of liquid investments		3 months
Restricted cash		$ 821,000	671,000
Total consideration for properties sold including seller financing	$ 15,000,000
Ramoco Affiliates [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of properties sold to Ramoco affiliates that were previously included in the Ramoco lease | Tenant		48
Number of leased properties | Property		61
Deferred gain on sale of property			3,868,000
Ramoco Affiliates [Member] | Accounts Payable and Accrued Liabilities [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Deferred gain on sale of property				$ 3,900,000
Seller Financing Mortgage [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Total consideration for properties sold including seller financing	$ 13,900,000
Level 3 [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Assumed holding periods for unobservable inputs		15 years
Assumed annual average rent increases for unobservable inputs		2.00%
Rate of income capitalization for unobservable inputs		8.00%
Cash flows discounted rate for unobservable inputs		7.00%
Estimated Sale Price Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges		$ 9,321,000
Estimated fair value		3,288,000
Discounted Cash Flow Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges		9,321,000
Estimated fair value		451,000
Accumulation of Asset Retirement Cost Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges		9,321,000
Estimated fair value		5,582,000
Notes and Mortgages Receivable [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges		0	0
Direct Financing Leases Financing Receivable [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges		$ 0	$ 0
Buildings and Improvements [Member] | Minimum [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Property plant and equipment useful life		16 years
Buildings and Improvements [Member] | Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Property plant and equipment useful life		25 years
USTs [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Property plant and equipment useful life		10 years