v3.8.0.1

Unsecured Credit Agreements - Summary of Unsecured Credit Agreements (Parenthetical) (Detail) - USD ($) | Dec. 31, 2017 | Nov. 20, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Revolving credit facility, available borrowing capacity |  | $ 880,000,000 |
Carrying amount of debt | $ 1,181,470,000 |  | $ 869,524,000
Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Carrying amount of debt | $ 1,113,000,000 |  | $ 762,000,000
Unsecured Debt | 1 Month LIBOR |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | 1.37% |  | 0.62%
Carrying amount of debt |  |  | $ 223,000,000
Unsecured Debt | 3 Month LIBOR |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate |  |  | 0.93%
Carrying amount of debt |  |  | $ 50,000,000
Unsecured Debt | 2012 Unsecured Credit Agreement |  |  |
Debt Instrument [Line Items] |  |  |
Carrying amount of debt |  |  | 202,000,000
Unsecured Debt | 2012 Unsecured Credit Agreement | Revolver |  |  |
Debt Instrument [Line Items] |  |  |
Revolving credit facility, available borrowing capacity |  |  | 273,200,000
Line of credit, maximum borrowing capacity |  |  | 300,000,000
Carrying amount of debt |  |  | $ 102,000,000
Unsecured Debt | 2013 Unsecured Credit Agreement |  |  |
Debt Instrument [Line Items] |  |  |
Leverage ratio |  |  | 1.75%
Carrying amount of debt |  |  | $ 185,000,000

X
- Definition Leverage ratio. + References No definition available. + Details
Name: | ck0001424182_LeverageRatio
Namespace Prefix: | ck0001424182_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
Name: | us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=us-gaap_UnsecuredDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=ck0001424182_OneMonthLondonInterbankOfferedRateLIBORMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=ck0001424182_ThreeMonthLondonInterbankOfferedRateLIBORMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ck0001424182_TwoThousandTwelveUnsecuredCreditAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditFacilityAxis=ck0001424182_RevolverMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ck0001424182_TwoThousandThirteenUnsecuredCreditAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |