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Mortgage Notes Payable and Credit Facility - Additional Information (Detail) |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  |  |  |  |
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Dec. 01, 2017 USD ($) | Oct. 27, 2017 USD ($) | May 26, 2016 USD ($) | Oct. 31, 2015 USD ($) | Jun. 30, 2018 USD ($) mortgage property | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) mortgage property | Jun. 30, 2018 USD ($) mortgage property | Jun. 30, 2018 USD ($) mortgage property | Jun. 30, 2018 USD ($) mortgage property | Jun. 30, 2017 USD ($) | Jun. 22, 2018 USD ($) | Apr. 18, 2018 USD ($) | Dec. 31, 2017 USD ($) | Jun. 22, 2017 | Oct. 05, 2015 USD ($) | Aug. 31, 2013 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties collateralized in mortgage notes payable | property |  |  |  |  | 98 |  | 98 | 98 | 98 | 98 |  |  |  |  |  |  |
Number of long-term mortgages repaid |  |  |  |  |  |  | 1 |  | 1 |  |  |  |  |  |  |  |
Number of properties repaid to collateralized mortgages notes payable | mortgage |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |
Number of properties collateralized | property |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |
Payments of deferred financing costs |  |  |  |  |  |  |  | $ 253,000 |  |  | $ 414,000 |  |  |  |  |  |
Interest rate swap |  |  |  |  | $ 1,115,000 |  | $ 1,115,000 | 1,115,000 | $ 1,115,000 | $ 1,115,000 |  |  |  | $ 504,000 |  |  |
Fair value of mortgage notes payable |  |  |  |  | 435,100,000 |  | 435,100,000 | 435,100,000 | 435,100,000 | 435,100,000 |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  |  |  |  |  |  |  |  | 75,000,000.0 |  |  | $ 25,000,000.0
Extension fee on initial maturity (as percent) |  | 0.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument fees incurred |  | $ 900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit outstanding |  |  |  |  | $ 32,808,000 |  | $ 32,808,000 | $ 32,808,000 | $ 32,808,000 | $ 32,808,000 |  |  |  | 20,715,000 |  |  |
Weighted average interest rate on debt outstanding (as percent) |  |  |  |  | 4.63% |  | 4.63% | 4.63% | 4.63% | 4.63% |  |  |  |  |  |  |
Term Loan Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of debt |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Extension fee on initial maturity (as percent) |  |  |  | 0.05% |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of debt |  | 4 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  | $ 85,000,000.0 |  |  |  |  |  |  |  |  |  |  |  | $ 85,000,000.0 |
Line of credit outstanding |  |  |  |  | $ 108,400,000 |  | $ 108,400,000 | $ 108,400,000 | $ 108,400,000 | $ 108,400,000 |  |  |  |  |  |  |
Weighted average interest rate (as percent) |  |  |  |  | 3.81% |  | 3.81% | 3.81% | 3.81% | 3.81% |  |  |  |  |  |  |
Letters of credit, outstanding |  |  |  |  | $ 1,000,000 |  | $ 1,000,000 | $ 1,000,000 | $ 1,000,000 | $ 1,000,000 |  |  |  |  |  |  |
Maximum additional amount that can be drawn under Line of Credit |  |  |  |  | $ 46,100,000 |  | $ 46,100,000 | $ 46,100,000 | $ 46,100,000 | $ 46,100,000 |  |  |  |  |  |  |
Line of credit | Term Loan Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  | $ 25,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Term of debt |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |
Letter of credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average interest rate on debt outstanding (as percent) |  |  |  |  | 1.75% |  | 1.75% | 1.75% | 1.75% | 1.75% |  |  |  |  |  |  |
Variable rate mortgage loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties collateralized in mortgage notes payable | property |  |  |  |  | 18 |  | 18 | 18 | 18 | 18 |  |  |  |  |  |  |
Carrying value |  |  |  |  | $ 61,590,000 |  | $ 61,590,000 | $ 61,590,000 | $ 61,590,000 | $ 61,590,000 |  |  | $ 13,000,000.0 | 69,302,000 |  |  |
Mortgage notes payable |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of mortgage notes payable | mortgage |  |  |  |  | 45 |  | 45 | 45 | 45 | 45 |  |  |  |  |  |  |
Number of properties collateralized in mortgage notes payable | property |  |  |  |  | 66 |  | 66 | 66 | 66 | 66 |  |  |  |  |  |  |
Net book value of collateralized mortgage properties |  |  |  |  | $ 639,400,000 |  | $ 639,400,000 | $ 639,400,000 | $ 639,400,000 | $ 639,400,000 |  |  |  |  |  |  |
Recourse amount of mortgage notes payable |  |  |  |  | 10,200,000 |  | 10,200,000 | 10,200,000 | 10,200,000 | $ 10,200,000 |  |  |  |  |  |  |
Percentage of debt recourse (as percent) |  |  |  |  |  |  |  |  |  | 2.30% |  |  |  |  |  |  |
Carrying value |  |  |  |  | $ 439,877,000 |  | $ 439,877,000 | $ 439,877,000 | $ 439,877,000 | $ 439,877,000 |  |  |  |  |  |  |
Weighted average interest rate on debt outstanding (as percent) |  |  |  |  | 4.46% |  | 4.46% | 4.46% | 4.46% | 4.46% |  |  |  |  |  |  |
Issued and assumed mortgage notes payable |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of mortgage notes payable | mortgage |  |  |  |  | 1 |  | 1 | 1 | 1 | 1 |  |  |  |  |  |  |
Number of properties collateralized in mortgage notes payable | property |  |  |  |  | 1 |  | 1 | 1 | 1 | 1 |  |  |  |  |  |  |
Fixed rate mortgage loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties collateralized in mortgage notes payable | property |  |  |  |  | 48 |  | 48 | 48 | 48 | 48 |  |  |  |  |  |  |
Payments of deferred financing costs |  |  |  |  | $ 30,000 | $ 400,000 |  | $ 200,000 |  |  | $ 400,000 |  |  |  |  |  |
Stated interest rate (as percent) |  |  |  |  | 5.75% |  | 5.75% | 5.75% | 5.75% | 5.75% |  |  |  |  |  |  |
Carrying value |  |  |  |  | $ 378,287,000 |  | $ 378,287,000 | $ 378,287,000 | $ 378,287,000 | $ 378,287,000 |  |  |  | $ 383,189,000 |  |  |
Conshohocken, Pennsylvania |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (as percent) |  |  |  |  |  |  |  |  |  |  |  | 1.80% |  |  | 1.80% |  |
Mortgage loans on real estate, new mortgage loans |  |  | $ 11,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Columbus, Ohio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate (as percent) | 2.83% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate swap | Conshohocken, Pennsylvania |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate swap |  |  |  |  | 700,000 |  | 700,000 | 700,000 | 700,000 | 700,000 |  | $ 0 |  |  |  |  |
Interest rate swap | Columbus, Ohio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate swap |  |  |  |  | $ (100,000) |  | $ (100,000) | $ (100,000) | $ (100,000) | $ (100,000) |  |  |  |  |  |  |
Debt assumed | $ 9,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
LIBOR |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spread on LIBOR (as percent) |  | 2.75% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
LIBOR | Variable rate mortgage loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spread on LIBOR (as percent) |  |  |  |  |  |  |  |  |  | 2.25% |  |  |  |  |  |  |

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- Definition Line of credit facility percentage of extension fee on initial maturity. + References No definition available. + Details
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- Definition Number Of Long Term Mortgages Repaid + References No definition available. + Details
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- Definition Number of mortgage notes payable. + References No definition available. + Details
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- Definition Number of Properties Repaid To Collateralized Mortgage Notes Payable + References No definition available. + Details
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- 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
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- Definition Amount of assets pledged to secure a debt instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823 + Details
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- Definition Amount of the fee that accompanies borrowing money under the debt instrument. + 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(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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 + Details
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- 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
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Weighted average interest rate of debt outstanding. + References No definition available. + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 + Details
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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + 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
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- Definition The effective interest rate at the end of the reporting period. + 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
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- 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
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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + 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
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- Definition Amount of the contractual principal due at the origination of the mortgage loan (face amount). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-29) -URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 10 -Section S99 -Paragraph 6 -Subparagraph (SX 210.5-04.(c) Schedule IV) -URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674 + Details
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- Definition Total amount of new mortgage loans on real estate during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-29.6) -URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 10 -Section S99 -Paragraph 6 -Subparagraph (SX 210.5-04.(c) Schedule IV) -URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674 + Details
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- Definition The amount of debt that an Entity assumes in acquiring a business or in consideration for an asset received in a noncash (or part noncash) acquisition. Noncash is defined as transactions during a period that affect recognized assets or liabilities but that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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- Definition Fair value portion of notes payable. + 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 210.5-02.19,20,22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The cash outflow for loan and debt issuance costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585 + Details
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