v3.8.0.1

Sale of Real Estate - Schedule of Sale of Real Estate (Detail) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) Property | Dec. 31, 2016 USD ($) Property | Dec. 31, 2015 USD ($) Property
Real Estate [Abstract] |  |  |
Number of properties disposed | Property | 13 | 9 | 6
Aggregate sale price | $ 66,532 | $ 39,500 | $ 19,455
Aggregate carrying value | 50,339 | 32,665 | 18,672
Additional sales expenses | 3,201 | 910 | 1,496
Gain (loss) on sale of real estate | $ 12,992 | $ 5,925 | $ (713)

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- Definition Additional expenses of rental property disposed. + References No definition available. + Details
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Namespace Prefix: | ck0001424182_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Carrying value of real estate, sold. + References No definition available. + Details
Name: | ck0001424182_CarryingValueOfRealEstateSold
Namespace Prefix: | ck0001424182_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Number of rental property disposed. + References No definition available. + Details
Name: | ck0001424182_NumberOfRentalPropertyDisposed
Namespace Prefix: | ck0001424182_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Cash received for the sale of real estate that is not part of an investing activity during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585 + Details
Name: | us-gaap_ProceedsFromSaleOfRealEstate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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