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Accounting Policies - Additional Information (Detail) | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) Property | Jun. 30, 2017 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items] |  |  |  |
Impairment charges | $ 1,160,000 | $ 914,000 | $ 3,977,000 | $ 4,651,000
Number of properties disposed | Property |  |  | 6 |
Gain on dispositions of real estate | 3,016,000 | 507,000 | $ 3,665,000 | 176,000
Level 3 [Member] |  |  |  |
Organization Consolidation And Presentation Of Financial Statements [Line Items] |  |  |  |
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% |
Level 3 [Member] | Maximum [Member] |  |  |  |
Organization Consolidation And Presentation Of Financial Statements [Line Items] |  |  |  |
Assumed holding periods for unobservable inputs |  |  | 15 years |
Estimated Sale Price Method [Member] |  |  |  |
Organization Consolidation And Presentation Of Financial Statements [Line Items] |  |  |  |
Impairment charges |  |  | $ 3,977,000 |
Estimated fair value | 2,338,000 |  | 2,338,000 |
Discounted Cash Flow Method [Member] |  |  |  |
Organization Consolidation And Presentation Of Financial Statements [Line Items] |  |  |  |
Impairment charges |  |  | 3,977,000 |
Estimated fair value | 104,000 |  | 104,000 |
Accumulation of Asset Retirement Cost Method [Member] |  |  |  |
Organization Consolidation And Presentation Of Financial Statements [Line Items] |  |  |  |
Impairment charges |  |  | 3,977,000 |
Estimated fair value | 1,535,000 |  | 1,535,000 |
Notes and Mortgages Receivable [Member] |  |  |  |
Organization Consolidation And Presentation Of Financial Statements [Line Items] |  |  |  |
Impairment charges | 0 | 0 | 0 | 0
Direct Financing Leases Financing Receivable [Member] |  |  |  |
Organization Consolidation And Presentation Of Financial Statements [Line Items] |  |  |  |
Impairment charges | $ 0 | $ 0 | $ 0 | $ 0

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