Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Parent	Preference shares Series B preference shares	Common shares	Additional paid-in capital	Retained earnings	Accumulated other comprehensive loss	Noncontrolling interests
Balance, beginning of period at Dec. 31, 2018			$ 0.0	$ 9.4	$ 918.9	$ 276.3	$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common shares, net			0.0
Issuance of common shares, net				0.1	1.8
Acquisition of Sirius Group				0.0	0.0
Issuance of common shares to related party				0.0	0.0
Share compensation					7.0
Net income	$ 200.6					200.6
Net (income) loss attributable to noncontrolling interests	0.0					0.0
Dividends on preference shares						0.0
Net change in foreign currency translation	0.0						0.0
Balance, end of period at Dec. 31, 2019	1,414.1	$ 1,414.1	0.0	9.5	927.7	476.9	0.0	$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common shares, net			0.0
Issuance of common shares, net				0.1	(0.4)
Acquisition of Sirius Group				0.0	0.0
Issuance of common shares to related party				0.0	0.0
Share compensation					6.6
Net income	143.8					143.8
Net (income) loss attributable to noncontrolling interests	(0.3)					(0.3)
Dividends on preference shares						0.0
Net change in foreign currency translation	0.0						0.0
Balance, end of period at Dec. 31, 2020	1,565.3	1,563.9	0.0	9.6	933.9	620.4	0.0	1.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common shares, net			200.0
Issuance of common shares, net				0.2	13.0
Acquisition of Sirius Group				5.8	589.7
Issuance of common shares to related party				0.6	48.0
Share compensation					38.1
Net income	55.8					55.8
Net (income) loss attributable to noncontrolling interests	2.3					2.3
Dividends on preference shares						(13.5)
Net change in foreign currency translation	(0.2)						(0.2)
Balance, end of period at Dec. 31, 2021	$ 2,503.3	$ 2,503.7	$ 200.0	$ 16.2	$ 1,622.7	$ 665.0	$ (0.2)	$ (0.4)