Debt and standby letters of credit facilities - Standby Letter of Credit Facilities (Details) kr in Billions	Sep. 30, 2020 USD ($)	Sep. 30, 2020 SEK (kr)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 SEK (kr)	Nov. 09, 2019 USD ($) letter_of_credit
Standby Letter of Credit Facilities
Number of standby letter of credit agreements renewed | letter_of_credit					2
Other secured letter of credit and trust arrangement | Sirius america insurance company
Standby Letter of Credit Facilities
Secured letter of credit and trust arrangements were collateralized by pledged assets and assets in trust	$ 47,900,000		$ 57,700,000
Other secured letter of credit and trust arrangement | Sirius bermuda insurance company ltd.
Standby Letter of Credit Facilities
Secured letter of credit and trust arrangements were collateralized by pledged assets and assets in trust	519,900,000		784,000,000.0
Sirius International | Nordea bank finland abp
Standby Letter of Credit Facilities
Borrowing capacity					$ 90,000,000
Sirius International | DNB bank ASA london branch
Standby Letter of Credit Facilities
Borrowing capacity					35,000,000
Sirius International | Standby letters of credit
Standby Letter of Credit Facilities
Borrowing capacity					125,000,000
Sirius International | Unsecured standby letters of credit | DNB bank ASA london branch
Standby Letter of Credit Facilities
Borrowing capacity					$ 25,000,000
Sirius International | Other secured letter of credit and trust arrangement
Standby Letter of Credit Facilities
Secured letter of credit and trust arrangements were collateralized by pledged assets and assets in trust	$ 488,300,000	kr 4.4	$ 363,300,000	kr 3.4