v3.20.2

Equity - Summary of Redemptions under Company's Share Redemption Program (Detail) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 USD ($) Stockholder shares | Jun. 30, 2020 shares | Jun. 30, 2019 USD ($) Stockholder shares
Equity [Abstract] |  |  |
Number of redemptions requested | Stockholder | 16 |  | 29
Number of shares | shares | 38,000 | 0 | 59,000
Aggregate redemption price | $ | $ 3,210 |  | $ 5,013

X
- Definition Aggregate redemption price of common stock. + References No definition available. + Details
Name: | ck1424182_AggregateRedemptionPriceOfCommonStock
Namespace Prefix: | ck1424182_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of redemptions requested. + References No definition available. + Details
Name: | ck1424182_NumberOfRedemptionsRequested
Namespace Prefix: | ck1424182_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Number of stock bought back by the entity at the exercise price or redemption price. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644 + Details
Name: | us-gaap_StockRedeemedOrCalledDuringPeriodShares
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration