Investments in Unconsolidated Real Estate Ventures - Summary of Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Equity Method Investments and Joint Ventures [Abstract]
Variable rate, weighted average interest rate	2.38%
Variable rate	$ 767,164	$ 629,479
Fixed rate, weighted average interest rate	4.00%
Fixed rate	$ 566,025	561,236
Unconsolidated real estate ventures - mortgages payable	1,333,189	1,190,715
Unamortized deferred financing costs	(8,138)	(2,859)
Unconsolidated real estate ventures - mortgages payable, net	$ 1,325,051	$ 1,187,856

Equity Method Investment, Long Term Debt

No definition available.

Name:	jbgs_EquityMethodInvestmentLongTermDebt
Namespace Prefix:	jbgs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Equity Method Investment, Long Term Debt, Bearing Fixed, Amount

No definition available.

Name:	jbgs_EquityMethodInvestmentLongTermDebtBearingFixedAmount
Namespace Prefix:	jbgs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Equity Method Investment, Long Term Debt, Bearing Variable, Amount

No definition available.

Name:	jbgs_EquityMethodInvestmentLongTermDebtBearingVariableAmount
Namespace Prefix:	jbgs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Equity Method Investment, Long Term Debt, Net

No definition available.

Name:	jbgs_EquityMethodInvestmentLongTermDebtNet
Namespace Prefix:	jbgs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Equity Method Investment, Long Term Debt, Percentage Bearing Fixed Interest, Percentage Rate

No definition available.

Name:	jbgs_EquityMethodInvestmentLongTermDebtPercentageBearingFixedInterestPercentageRate
Namespace Prefix:	jbgs_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Equity Method Investment, Long Term Debt, Percentage Bearing Variable Interest, Percentage Rate

No definition available.

Name:	jbgs_EquityMethodInvestmentLongTermDebtPercentageBearingVariableInterestPercentageRate
Namespace Prefix:	jbgs_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Equity Method Investment, Unamortized Deferred Financing Costs

No definition available.

Name:	jbgs_EquityMethodInvestmentUnamortizedDeferredFinancingCosts
Namespace Prefix:	jbgs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

No definition available.

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