Summary of significant accounting policies - Federal and foreign income taxes (Details)	12 Months Ended
Dec. 31, 2018
Summary of significant accounting policies
Tax Cuts and Jobs Act of 2017, BEAT tax rate, 2018 (as a percent)	5.00%
Tax Cuts and Jobs Act of 2017, BEAT tax rate, 2019-2025 (as a percent)	10.00%
Tax Cuts and Jobs Act of 2017, BEAT tax rate, 2026 and thereafter (as a percent)	12.50%

Percentage of base erosion and anti-abuse tax rate (BEAT) in year eight and thereafter under the Tax Cut and Jobs Act, 2017.

No definition available.

Name:	sg_TaxCutAndJobsAct2017BaseErosionAndAntiAbuseTaxRateYearEightAndThereafter
Namespace Prefix:	sg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration