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Derivatives - Summary of Terms of Derivative Financial Instrument (Details) - Cap Agreement - USD ($) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2020 | Dec. 31, 2019
Derivative [Line Items] |  |
Notional Amount | $ 7,500,000 | $ 7,500,000
Effective Date | Dec. 24, 2018 |
Maturity Date | Dec. 20, 2021 |
Minimum |  |
Derivative [Line Items] |  |
Strike | 3.50% |
Maximum |  |
Derivative [Line Items] |  |
Strike | 4.00% |

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- Definition Derivative strike price percentage. + References No definition available. + Details
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Namespace Prefix: | self_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Date the entity entered into the derivative contract, in CCYY-MM-DD format. + References No definition available. + Details
Name: | us-gaap_DerivativeInceptionDates
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Date the derivative contract matures, in CCYY-MM-DD format. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Nominal or face amount used to calculate payment on derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 + Details
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Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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