Employee benefit plans and compensation plans - Defined benefit plans - Obligations (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Sweden
Employee benefit plans and compensation plans
Discount rate	2.30%	2.50%
Increase in compensation levels rate	3.30%	3.20%
Turnover rate	3.00%	3.00%
Additional information
Pension liability (in years)	20 years
Germany
Employee benefit plans and compensation plans
Discount rate	1.90%	1.70%
Increase in compensation levels rate	2.00%	2.00%
Additional information
Pension liability (in years)	17 years

Represents the duration (in years) of the pension liability.

No definition available.

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

- Definition

Turn over rate used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to pay-related defined benefit plan.

+ References

No definition available.

+ Details

Name:	sg_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationTurnoverRate
Namespace Prefix:	sg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(1)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(2)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_SE
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_DE
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: