Business combination recognized identifiable assets acquired and liabilities assumed mortgage loans.

No definition available.

Name:	ck1368757_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedMortgageLoans
Namespace Prefix:	ck1368757_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Debt instrument interest rate over the original interest rate up on default.

+ References

No definition available.

+ Details

Name:	ck1368757_DebtInstrumentInterestRateOverTheOriginalInterestRateUponDefault
Namespace Prefix:	ck1368757_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument interest rate upon default.

+ References

No definition available.

+ Details

Name:	ck1368757_DebtInstrumentInterestRateUponDefault
Namespace Prefix:	ck1368757_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Increase in debt instrument interest rate upon default.

+ References

No definition available.

+ Details

Name:	ck1368757_IncreaseInDebtInstrumentInterestRateUponDefault
Namespace Prefix:	ck1368757_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Loan Prepayment Premium Percentage calculated on the outstanding principal amount of loan facility or the then present value of the note in event of prepayment.

+ References

No definition available.

+ Details

Name:	ck1368757_LoanPrepaymentPremiumPercentage
Namespace Prefix:	ck1368757_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of collateralized properties.

+ References

No definition available.

+ Details

Name:	ck1368757_NumberOfCollateralizedProperties
Namespace Prefix:	ck1368757_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of properties acquired.

+ References

No definition available.

+ Details

Name:	ck1368757_NumberOfPropertiesAcquired
Namespace Prefix:	ck1368757_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of wholly owned subsidiaries.

+ References

No definition available.

+ Details

Name:	ck1368757_NumberOfWhollyOwnedSubsidiaries
Namespace Prefix:	ck1368757_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Maturity date of mortgage loan within investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Paragraph Column C
-Subsection 29

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Column C))
-URI http://asc.fasb.org/extlink&oid=120402547&loc=d3e617274-123014

+ Details

Name:	srt_MortgageLoanOnRealEstateFinalMaturityDate
Namespace Prefix:	srt_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest rate of mortgage loan within investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Paragraph Column B
-Subsection 29

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Column B))
-URI http://asc.fasb.org/extlink&oid=120402547&loc=d3e617274-123014

+ Details

Name:	srt_MortgageLoansOnRealEstateInterestRate
Namespace Prefix:	srt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

Remaining amortization period for discount on the liability component of convertible debt which may be settled in cash upon conversion, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031898-161870

+ Details

Name:	us-gaap_DebtInstrumentConvertibleRemainingDiscountAmortizationPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The average effective interest rate during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(3))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentPaymentTerms
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of payment greater than the preceding installment payments to be paid at final maturity date of debt.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Discussion of the facts and amounts pertaining to each failure to comply with an affirmative or negative covenant of a long-term debt instrument, including violating payment terms or an inability to meet certain minimum financial requirements or achieve or maintain certain financial ratios. The discussion would generally be expected to also include whether or not the failure can and will be overcome and a description of the terms of any waivers, including the amount of the waiver and the period of time covered by the waiver, and if reclassification of long-term debt to current has been made in the current balance sheet.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(c))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6802200&loc=SL6230698-112601

+ Details

Name:	us-gaap_DefaultLongtermDebtDescriptionOfViolationOrEventOfDefault
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Expenses paid for obtaining loans which includes expenses such as application and origination fees.

+ References

No definition available.

+ Details

Name:	us-gaap_LoanProcessingFee
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

+ Details

Name:	us-gaap_RepaymentsOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_SecuredDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=ck1368757_AllstateLifeInsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=ck1368757_ThreePointEightTwoPercentInterestUnitedStatesLifeInsuranceCompanyLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ck1368757_EventOfDefaultMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=ck1368757_OriginalInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=ck1368757_FourPointTwoFivePercentInterestUnitedStatesLifeInsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=ck1368757_ThreePointFourFiveTransamericaLifeInsuranceCompanyLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=ck1368757_NoteRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=ck1368757_PiscatawayNewJerseyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=ck1368757_AmericanInternationalGroupLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=ck1368757_PeoplesUnitedBankLoanAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=ck1368757_HartfordAccidentAndIndemnityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=ck1368757_PeoplesUnitedBankLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Mortgage Notes Payable - Additional Information (Detail)									1 Months Ended		6 Months Ended	12 Months Ended
	Sep. 23, 2020 USD ($)	Mar. 24, 2020 USD ($) Item	Feb. 25, 2020 USD ($)	Mar. 21, 2018 USD ($) Item	Dec. 20, 2017 USD ($) Property Item	Mar. 13, 2015 USD ($)	Feb. 20, 2015 USD ($) Property	Sep. 30, 2010 USD ($)	Feb. 28, 2017 USD ($)	Apr. 30, 2014 USD ($)	Jun. 30, 2021 USD ($) Property	Dec. 31, 2014 USD ($) ft²	Dec. 31, 2020 USD ($)	Dec. 31, 2017
Debt Instrument [Line Items]
Mortgage notes payable											$ 365,821,000		$ 366,201,000
Number of wholly-owned subsidiaries of the UPREIT \| Item		2		4	4
Repayments of outstanding indebtedness			$ 6,000,000.0
Allstate Life Insurance Company, Loan [Member]
Debt Instrument [Line Items]
Mortgage notes payable											$ 36,821,000		37,201,000
Interest Rate											4.00%
Maturity											Apr. 01, 2025
3.82 % United States Life Insurance Company, Loan [Member]
Debt Instrument [Line Items]
Mortgage notes payable					$ 39,000,000.0						$ 39,000,000		39,000,000
Permanent financing period					10 years
Debt instrument, payment terms											During the period from February 1, 2018 to December 1, 2027, payments of interest only on the principal balance of the U.S. Life Note (as defined below) will be payable in arrears, with the entire principal balance due and payable on January 1, 2028, the loan maturity date.
Number of collateralized properties \| Property					4
Interest Rate					3.82%						3.82%
Maturity					Jan. 01, 2028						Jan. 01, 2028
Application fee to lender					$ 50,000
Event of default, description											In the event of default, the initial rate of interest on the U.S. Life Note will increase to the greatest of (i) 18% per annum, (ii) a per annum rate equal to 4% over the prime established rate, or (iii) a per annum rate equal to 5% over the original interest rate, all subject to the applicable state or federal laws.
3.82 % United States Life Insurance Company, Loan [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Loan Prepayment Premium Percentage,upon providing advance notice of prepayment					1.00%
3.82 % United States Life Insurance Company, Loan [Member] \| Minimum [Member] \| Event of Default [Member]
Debt Instrument [Line Items]
Increase in debt instrument interest rate upon default					18.00%
3.82 % United States Life Insurance Company, Loan [Member] \| Prime Rate [Member] \| Event of Default [Member]
Debt Instrument [Line Items]
Debt instrument Interest rate upon default					4.00%
3.82 % United States Life Insurance Company, Loan [Member] \| Original Interest Rate [Member] \| Event of Default [Member]
Debt Instrument [Line Items]
Debt instrument interest rate over original interest rate upon default					5.00%
4.25 % United States Life Insurance Company, Loan [Member]
Debt Instrument [Line Items]
Mortgage notes payable				$ 33,000,000.0							$ 33,000,000		$ 33,000,000
Permanent financing interest rate				4.25%
Permanent financing period				10 years
Debt instrument, payment terms											the principal balance for the first five years of the term and principal and interest payments (amortized over a 30-year period) during the second five years of the term. The entire principal balance is due and payable on April 1, 2028, the loan maturity date.
Interest Rate											4.25%
Maturity											Apr. 01, 2028
3.45% Transamerica Life Insurance Company, Loan [Member]
Debt Instrument [Line Items]
Mortgage notes payable		$ 8,400,000
Permanent financing interest rate		3.45%
Permanent financing period		10 years
Debt instrument, payment terms											principal balance of the note until April 1, 2022 and (ii) principal and interest payments (amortized over a 25-year period commencing at the end of the interest-only period) from May 1, 2022 through March 1, 2030. The entire principal balance of each note is due and payable on April 1, 2030, the loan maturity date
3.45% Transamerica Life Insurance Company, Loan [Member] \| Event of Default [Member]
Debt Instrument [Line Items]
Event of default, description											If a default exists, the outstanding principal balance of the notes shall, at the option of the lender, bear interest at a rate equal to the lesser of (i) 10% per annum over the note rate and (ii) the highest rate of interest permitted to be paid or collected by applicable law with respect to the loan.
3.45% Transamerica Life Insurance Company, Loan [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Loan Prepayment Premium Percentage,upon providing advance notice of prepayment		1.00%
3.45% Transamerica Life Insurance Company, Loan [Member] \| Note Rate [Member] \| Event of Default [Member]
Debt Instrument [Line Items]
Debt instrument Interest rate upon default		10.00%
Piscataway, NJ [Member]
Debt Instrument [Line Items]
Number of properties acquired \| Property											6
Piscataway, NJ [Member] \| Allstate Life Insurance Company, Loan [Member]
Debt Instrument [Line Items]
Mortgage notes payable											$ 39,100,000
Permanent financing period						10 years
Payment term based on amortization schedule						30 years
Debt Instrument, balloon payment due upon maturity						$ 33,700,000
Interest Rate						4.00%
Maturity						Apr. 01, 2025
AIG Loan [Member]
Debt Instrument [Line Items]
Mortgage notes payable							$ 233,100,000
Permanent financing interest rate							4.05%
Permanent financing period							10 years
Debt instrument, payment terms											During the period from April 1, 2015, to February 1, 2025, payments of interest-only will be payable in arrears with the entire principal balance plus any accrued and unpaid interest due and payable on March 1, 2025.
Number of collateralized properties \| Property							28
People's United Bank Loan Agreement [Member]
Debt Instrument [Line Items]
Leasable area owned by the company \| ft²												84,000
Mortgage notes payable												$ 15,500,000
Permanent financing interest rate												4.18%
Permanent financing period												10 years
Payment term based on amortization schedule												25 years
Debt instrument, payment terms											Payments for the first seven years are interest only. Payments over the remaining three years of the term are based on a 25-year amortization schedule, with a balloon payment of $14.4 million due at maturity
Debt Instrument, balloon payment due upon maturity												$ 14,400,000
Hartford Accident & Indemnity Loan [Member]
Debt Instrument [Line Items]
Business acquisition, assumed mortgage										$ 9,000,000.0
Mortgage, bears interest rate									5.20%	6.07%
Repayments of outstanding indebtedness									$ 3,000,000.0
Mortgage, maturity date											Feb. 25, 2020
People's United Bank Loan [Member]
Debt Instrument [Line Items]
Interest Rate														5.23%
Maturity											Oct. 01, 2020
Repayments of outstanding indebtedness	$ 2,000,000.0
New borrowings								$ 2,700,000