v3.19.3.a.u2

Summary of Significant Accounting Policies - Summary of Tenant and Capital Reserves (Detail) - Accounts Payable and Other Liabilities [Member] - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Significant Accounting Policies [Line Items] |  |
Tenant reserve | $ 922 | $ 774
Capital reserve | 872 | 362
Tenant and capital reserves | $ 1,794 | $ 1,136

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- Definition Capital reserves. + References No definition available. + Details
Name: | ck1424182_CapitalReserves
Namespace Prefix: | ck1424182_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Significant accounting policies. + References No definition available. + Details
Name: | ck1424182_SignificantAccountingPoliciesLineItems
Namespace Prefix: | ck1424182_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Tenant and capital reserves. + References No definition available. + Details
Name: | ck1424182_TenantAndCapitalReserves
Namespace Prefix: | ck1424182_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Carrying amount as of the balance sheet date of improvements having a life longer than one year that were made for the benefit of one or more tenants. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 + Details
Name: | us-gaap_TenantImprovements
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_BalanceSheetLocationAxis=ck1424182_AccountsPayableAndOtherLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |