The amount of retention bonuses recognized during the period pertaining to the sale of the entity.

No definition available.

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

- Definition

The amount of reimbursements for retention bonuses received during the period from former parent in relation to the sale of the entity.

+ References

No definition available.

+ Details

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

- Definition

The amount of transaction bonuses recognized during the period pertaining to the sale of the entity.

+ References

No definition available.

+ Details

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

- Definition

Amount of incentive awards paid to employees received from former parent, under share-based arrangement compensation.

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment made to pay deferred cash remuneration.

+ References

No definition available.

+ Details

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

- Definition

Amount of distribution made to individual in accordance with deferred compensation arrangement.

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualDistributionPaid
Namespace Prefix:	us-gaap_
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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

- Details

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Namespace Prefix:
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Period Type:

Employee benefit plans and compensation plans - Transaction and retention bonuses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Transaction bonuses recognized			$ 36.9
Amount received from former parent for transaction and retention bonuses		$ 25.3	30.5
Retention bonuses recorded		13.3	17.6
2016 Long Term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of incentive awards paid to employees received from White Mountains		$ 1.4	$ 2.5
Retention awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Retention awards issued	$ 10.7
Retention awards recognized	$ 2.1