Commitments and Contingencies - Summary of Future Minimum Rental Payments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2018	$ 13,686
2019	14,073
2020	13,866
2021	13,594
2022	12,845
Thereafter	697,903
Total	$ 765,967

Operating And Capital Leases, And Lease Assumption Liabilities, Future Minimum Payments Due

No definition available.

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

Operating And Capital Leases, And Lease Assumption Liabilities, Future Minimum Payments, Due In Five Years

No definition available.

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

Operating And Capital Leases, And Lease Assumption Liabilities, Future Minimum Payments, Due In Four Years

No definition available.

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

Operating And Capital Leases, And Lease Assumption Liabilities, Future Minimum Payments Due, Next Twelve Months

No definition available.

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

Operating And Capital Leases, And Lease Assumption Liabilities, Future Minimum Payments, Due In Three Years

No definition available.

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

Operating And Capital Leases, And Lease Assumption Liabilities, Future Minimum Payments, Due In Two Years

No definition available.

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

Operating And Capital Leases, And Lease Assumption Liabilities, Future Minimum Payments, Due Thereafter

No definition available.

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

No definition available.

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