v3.19.3.a.u2

Income Taxes - Schedule of Distributions Characterized as Ordinary Dividends Capital Gain and Nontaxable (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Income Tax Disclosure [Abstract] |  |  |
Ordinary dividends | 43.00% | 55.00% | 51.00%
Capital gain distributions | 8.00% | 8.00% |
Return of capital distributions | 49.00% | 37.00% | 49.00%
Percentage of distributions paid to stockholders | 100.00% | 100.00% | 100.00%

X
- Definition Percentage of capital gain distributions. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of distributions paid to stockholders. + References No definition available. + Details
Name: | ck1424182_PercentageOfDistributionsPaidToStockholders
Namespace Prefix: | ck1424182_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of ordinary dividends. + References No definition available. + Details
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Namespace Prefix: | ck1424182_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Return of capital distributions percentage. + References No definition available. + Details
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Namespace Prefix: | ck1424182_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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