Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Oct. 31, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	001-38731
Entity Registrant Name	SIRIUS INTERNATIONAL INSURANCE GROUP, LTD.
Entity Incorporation, State or Country Code	D0
Entity Tax Identification Number	98-0529995
Entity Address, Address Line One	14 Wesley Street
Entity Address, City or Town	Hamilton
Entity Address, Postal Zip Code	HM 11
Entity Address, Country	BM
City Area Code	441
Local Phone Number	278-3140
Title of 12(b) Security	Common shares, par value $0.01 per share
Trading Symbol	SG
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		115,299,341
Entity Central Index Key	0001744894
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3