Segment Information - Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 557.8	$ 581.5	$ 1,328.0	$ 1,244.4
Commission and fee revenues	82.3	46.8	193.7	108.4
Other revenues	68.8	14.1	138.7	42.3
Total	708.9	642.4	1,660.4	1,395.1
United States
Segment Reporting Information [Line Items]
Earned insurance premiums	5.2	0.0	21.7	2.8
Commission and fee revenues	78.0	42.7	181.3	97.3
Other revenues	68.8	14.1	138.7	42.3
Total	152.0	56.8	341.7	142.4
United Kingdom
Segment Reporting Information [Line Items]
Earned insurance premiums	335.2	326.4	779.7	697.5
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.0	0.0	0.0
Total	335.2	326.4	779.7	697.5
Bermuda
Segment Reporting Information [Line Items]
Earned insurance premiums	217.4	255.1	526.6	544.1
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.0	0.0	0.0
Total	217.4	255.1	526.6	544.1
Other
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Commission and fee revenues	4.3	4.1	12.4	11.1
Other revenues	0.0	0.0	0.0	0.0
Total	$ 4.3	$ 4.1	$ 12.4	$ 11.1