The amount of industry loss warranty contract retention limit.

No definition available.

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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

Reinsurance Retention Policy, Quota Share Treaty, Percentage

+ References

No definition available.

+ Details

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Namespace Prefix:	sg_
Data Type:	num:percentItemType
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

Amount before valuation allowance of reinsurance recoverables on paid losses.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, and for amounts expected based upon statistical projections and other measures of incurred losses and loss settlement expenses which have not yet been reported to the ceding insurer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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_ExcessOfLossReinsuranceCoverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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:	us-gaap_StatementBusinessSegmentsAxis=sg_GlobalReinsuranceMember
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Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
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- Details

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Data Type:	na
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- Details

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Third party reinsurance - Industry loss warranty contracts and Narrative (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) event	Dec. 31, 2018 USD ($)
Third party reinsurance
Reinsurance recoverable on paid losses	$ 73,900,000	$ 55,000,000
Reinsurance recoverable on unpaid losses	$ 410,300,000	$ 350,200,000
Excess of loss reinsurance coverage
Third party reinsurance
Cost coverage of original insurance premium (as a percent)	100.00%
Industry loss warranty contract
Third party reinsurance
Number of loss events covered \| event	1
Industry loss warranty contract \| United States excluding North East \| All natural perils
Third party reinsurance
Limit	$ 5,000,000
Trigger	$ 40,000,000,000
Global Reinsurance
Third party reinsurance
Quota share treaty, percentage	12.50%
Global Reinsurance \| Industry loss warranty contract
Third party reinsurance
Limit	$ 27,500,000
Global Reinsurance \| Minimum \| Excess of loss reinsurance coverage
Third party reinsurance
Reinsurance protection in excess of retention	150,000,000.0
Global Reinsurance \| Minimum \| Industry loss warranty contract
Third party reinsurance
Reinsurance protection in excess of retention	400,000,000
Global Reinsurance \| Maximum \| Excess of loss reinsurance coverage
Third party reinsurance
Reinsurance protection in excess of retention	2,000,000,000.0
Global Reinsurance \| Maximum \| Industry loss warranty contract
Third party reinsurance
Reinsurance protection in excess of retention	800,000,000
Global A&H \| Excess of loss reinsurance coverage
Third party reinsurance
Retention amount	$ 1,500,000