The amount of industry loss warranty contract retention limit.

No definition available.

Name:	sg_IndustryLossWarrantyContractsLimit
Namespace Prefix:	sg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The industry loss warranty contract (ILW), industry loss trigger amount.

+ References

No definition available.

+ Details

Name:	sg_IndustryLossWarrantyContractsTrigger
Namespace Prefix:	sg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of loss events covered under excess of loss reinsurance protections.

+ References

No definition available.

+ Details

Name:	sg_NumberOfLossEventCovered
Namespace Prefix:	sg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Provision in reinsurance contract related to percentage of cost coverage of original reinsurance premium that would be reinstated.

+ References

No definition available.

+ Details

Name:	sg_PercentageOfCostCoverageOfOriginalInsurancePremium
Namespace Prefix:	sg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of reinsurance retention policy excess retention amount reinsured.

+ References

No definition available.

+ Details

Name:	sg_PercentageOfReinsuranceRetentionPolicyExcessRetentionAmountReinsured
Namespace Prefix:	sg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Term of placement for purchase of excess of loss reinsurance protection, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	sg_TermOfPlacementForPurchaseOfExcessOfLossReinsuranceProtection
Namespace Prefix:	sg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Stated amount that is the upper retention limit per entity policy.

+ References

No definition available.

+ Details

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

- Definition

Amount of risk reinsured in excess of stated retention per company policy.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=sg_IndustryLossWarrantyReinsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_ProductOrServiceAxis=sg_PropertyLiabilityAndCasualtyInsuranceAllNaturalPerilsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_ProductOrServiceAxis=sg_PropertyLiabilityCasualtyInsuranceProductLineWindMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=sg_ExcessOfLossReinsuranceCoverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=sg_BusinessWrittenBySyndicate1945Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=sg_PropertyLiabilityCasualtyInsuranceProductLineCatastropheMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Third party reinsurance- Industry loss warranty contracts (Details) $ in Millions		12 Months Ended
	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($) item	Jun. 30, 2018 USD ($)
Industry loss warranty contract
Third party reinsurance
Number of loss events covered \| item		1
Industry loss warranty contract \| United States excluding North East \| All natural perils
Third party reinsurance
Limit		$ 5.0
Trigger		40,000.0
Industry loss warranty contract \| Europe \| Wind
Third party reinsurance
Limit		5.0
Trigger		$ 7,500.0
Excess of loss reinsurance coverage
Third party reinsurance
Cost coverage of original insurance premium (as a percent)		100.00%
Excess of loss reinsurance coverage \| Business written by Syndicate 1945
Third party reinsurance
Reinsurance protection in excess of retention			$ 10.0
Retention amount			$ 5.0
Term of placement (in months)			12 months
Excess of loss reinsurance coverage \| Catastrophe losses \| Business written by Syndicate 1945
Third party reinsurance
Reinsurance protection in excess of retention		$ 12.0
Retention amount		$ 3.0
Term of placement (in months)		12 months
Reinsurance protection in excess of retention (as a percent)		50.00%
Excess of loss reinsurance coverage \| Business written in Stockholm, Hamburg and Singapore
Third party reinsurance
Reinsurance protection in excess of retention	$ 32.5
Retention amount	$ 2.5