v3.21.2

Acquisitions of Rental Property - Additional Information - (Detail) - Property | Sep. 30, 2021 | Sep. 30, 2020
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Business Combinations [Abstract] |  |
Number of rental properties acquired | 80 | 0

X
- Definition The number​ of rental​ properties acquired. + References No definition available. + Details
Name: | bnl_NumberOfRentalPropertiesAcquired
Namespace Prefix: | bnl_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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