v3.8.0.1

Interest Rate Swaps - Summary of Interest-rate Swap Agreements (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2017
Interest Rate Swap | M&T Bank One | Umbrella Partnership Real Estate Investment Trust Transaction |
Derivative [Line Items] |
Interest rate swap assumed month and year | 2017-10

X
- Definition Interest rate swap assumed month and year. + References No definition available. + Details
Name: | ck0001424182_InterestRateSwapAssumedMonthAndYear
Namespace Prefix: | ck0001424182_
Data Type: | xbrli:gYearMonthItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CounterpartyNameAxis=ck0001424182_MAndTBankOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | invest_InvestmentAxis=ck0001424182_UmbrellaPartnershipRealEstateInvestmentTrustTransactionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |