v3.19.3.a.u2

Mortgages and Notes Payable - Summary of Extinguished Mortgages (Detail) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 USD ($) Mortgage | Dec. 31, 2018 USD ($) Mortgage | Dec. 31, 2017 USD ($) Mortgage
Debt Disclosure [Abstract] |  |  |
Number of mortgages | Mortgage | 4 | 2 | 7
Outstanding balance of mortgages | $ | $ 13,905 | $ 6,666 | $ 48,108

X
- Definition Number of mortgages extinguished. + References No definition available. + Details
Name: | ck1424182_NumberOfMortgagesExtinguished
Namespace Prefix: | ck1424182_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Gross amount of debt extinguished. + References No definition available. + Details
Name: | us-gaap_ExtinguishmentOfDebtAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration