Debt and standby letters of credit facilities - Standby Letter of Credit Facilities (Details) kr in Billions	Mar. 31, 2020 USD ($)	Mar. 31, 2020 SEK (kr)	Dec. 31, 2019 USD ($)	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	$ 47,800,000		$ 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	513,800,000		784,000,000
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	$ 335,600,000	kr 3.4	$ 363,300,000	kr 3.4