The amount of additional paid-in capital (APIC) from a former parent related to reimbursement of ILW covers.

No definition available.

Name:	sg_CapitalContributionsFromParentByReimbursementForIlwCovers
Namespace Prefix:	sg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of industry loss warranty contract retention limit.

+ References

No definition available.

+ Details

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

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

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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

Name:	us-gaap_ReinsuranceRecoverablesOnPaidLossesGross
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

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

- Details

Name:	dei_LegalEntityAxis=sg_CMIGInternationalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=sg_UnitedStatesFirstEventWithLimitOf75MillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=sg_UnitedStatesFirstEventWithLimitOf22.5MillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=sg_UnitedStatesSecondEventExpireOnJuly152016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=sg_JapanFirstEventMember
Namespace Prefix:
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Period Type:

Third party reinsurance- CMIG International acquisition (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Third party reinsurance
Reinsurance recoverable on paid losses		$ 55.0	$ 17.5
Reinsurance recoverable on unpaid losses		$ 350.2	$ 319.7
CMIG International
Third party reinsurance
Paid-in surplus	$ 16.5
United States first event with limit of $75 million \| CMIG International
Third party reinsurance
Limit	75.0
Industry Loss Trigger	40,000.0
United States first event with limit of $22.5 Million \| CMIG International
Third party reinsurance
Limit	22.5
Industry Loss Trigger	50,000.0
United States second event \| CMIG International
Third party reinsurance
Limit	45.0
Industry Loss Trigger	15,000.0
United States second event expire on July 15, 2016 \| CMIG International
Third party reinsurance
Limit	5.0
Japan first event \| CMIG International
Third party reinsurance
Limit	25.0
Industry Loss Trigger	$ 12,500.0