Shareholders' equity - Common shares issued and outstanding (Details) - shares	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Movement In Common Stock Outstanding [Roll Forward]
Common shares issued, beginning of period	95,582,733	94,225,498
Options exercised	220,000
Issuance of common shares for Sirius Group acquisition	58,331,196	0
Issuance of common shares to related party	6,093,842	0
Common shares issued, end of period	161,945,750	94,920,203
Options
Movement In Common Stock Outstanding [Roll Forward]
Options exercised		0
Restricted share awards
Movement In Common Stock Outstanding [Roll Forward]
Shares granted, net of forfeitures and shares withheld	3,182,511	350,409
Performance shares
Movement In Common Stock Outstanding [Roll Forward]
Shares granted, net of forfeitures and shares withheld	(1,464,532)	344,296