Significant transactions - Loss portfolio transfer (Details) $ in Millions	Jan. 23, 2020 USD ($)
Significant transactions
Cash received in loss portfolio transfer agreement	$ 69.7

Cash Received In Loss Portfolio Transfer Agreement

No definition available.

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