v3.8.0.1

Income Taxes - Schedule of Distributions Characterized as Ordinary Dividends Capital Gain and Nontaxable (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Tax Disclosure [Abstract] |  |  |
Ordinary dividends | 51.00% | 58.00% | 51.00%
Return of capital distributions | 49.00% | 42.00% | 49.00%
Percentage of distributions paid to stockholders | 100.00% | 100.00% | 100.00%

X
- Definition Percentage of distributions paid to stockholders. + References No definition available. + Details
Name: | ck0001424182_PercentageOfDistributionsPaidToStockholders
Namespace Prefix: | ck0001424182_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of ordinary dividends. + References No definition available. + Details
Name: | ck0001424182_PercentageOfOrdinaryDividends
Namespace Prefix: | ck0001424182_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Return of capital distributions percentage. + References No definition available. + Details
Name: | ck0001424182_ReturnOfCapitalDistributionsPercentage
Namespace Prefix: | ck0001424182_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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