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

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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