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Value of input used to measure long lived assets.

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The maximum original maturity period for which highly liquid investments will be considered cash and cash equivalents.

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The aggregate number of properties previously leased to subsidiaries of our tenant.

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The number of properties sold to tenant affiliates that were previously included in the tenant's lease.

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Organization consolidation and presentation of financial statements.

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Represents the percentage of annual assumed average rent increases used as an unobservable input to measure the fair value.

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Property plant and equipment assumed holding periods.

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Amount of gain on the sale of property that does not qualify for gain recognition as of the balance sheet date.

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The gain (loss) is the pre-tax difference between the net condemnation award and the entity's adjusted basis of the condemned property.

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Present value of lessee's discounted obligation for lease payments from operating lease.

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Amount of lessee's right to use underlying asset under operating lease.

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The total amount of revenue recognized for the period from operating leases, including minimum lease revenue, contingent revenue, percentage revenue and sublease revenue.

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Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Summary of Significant Accounting Policies - Additional Information (Detail)		3 Months Ended								12 Months Ended
	Aug. 03, 2015 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Property Tenant	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2015 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments										$ 4,902,000	$ 8,279,000	$ 8,566,000
Maturity period of liquid investments										3 months
Restricted cash		$ 1,850,000				$ 821,000				$ 1,850,000	821,000
Gain on property condemnations										60,000	16,000
Impairment charges										6,170,000	9,321,000	12,814,000
Revenues from rental properties	$ 15,000,000	$ 29,452,000	$ 29,570,000	$ 29,022,000	$ 28,284,000	28,158,000	$ 24,913,000	$ 24,364,000	$ 23,897,000	$ 116,328,000	101,332,000	$ 96,711,000
Accounting Standards Update 2016-02 [Member] \| Scenario Forecast [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
operating lease liabilities													$ 22,000,000
Operating lease right of use assets													$ 32,000,000
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
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]
Revenues from rental properties	$ 13,900,000
Level 3 [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Assumed annual average rent increases for unobservable inputs										2.00%
Level 3 [Member] \| Measurement Input Cap Rate [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Real estate fair value, measurement input		8.00%								8.00%
Level 3 [Member] \| Measurement Input Discount Rate [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Real estate fair value, measurement input		7.00%								7.00%
Estimated Sale Price Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges										$ 6,170,000
Estimated fair value						2,553,000					2,553,000
Discounted Cash Flow Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges										6,170,000
Estimated fair value						175,000					175,000
Accumulation of Asset Retirement Cost Method [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairment charges										6,170,000
Estimated fair value						$ 3,442,000					3,442,000
Notes and Mortgages Receivable [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments										0	0
Direct Financing Leases Financing Receivable [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Impairments										$ 0	$ 0
Maximum [Member] \| Level 3 [Member] \| Measurement Input Expected Term [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Assumed holding periods for unobservable inputs										15 years
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