Shareholders' equity - Common shares issued and outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Movement In Common Stock Issued and Outstanding [Roll Forward]
Common shares issued, beginning of year	95,582,733	94,225,498	93,639,610
Common shares outstanding, beginning of year	95,582,733	94,225,498	93,639,610
Options exercised	220,000	0	187,678
Issuance of common shares for Sirius Group acquisition	58,331,196	0	0
Issuance of common shares to related party	6,093,842	0	0
Common shares issued, end of year	161,929,777	95,582,733	94,225,498
Common shares outstanding, end of year	161,929,777	95,582,733	94,225,498
Restricted share awards
Movement In Common Stock Issued and Outstanding [Roll Forward]
Shares granted, net of forfeitures and shares withheld	3,133,969	1,012,939	366,453
Performance shares
Movement In Common Stock Issued and Outstanding [Roll Forward]
Shares granted, net of forfeitures and shares withheld	(1,431,963)	344,296	31,757