Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred acquisition costs and value of business acquired, net	$ 218.8	$ 68.6	$ 92.2
Loss and loss adjustment expense reserves	4,841.4	1,310.1	1,111.7
Unearned premium	1,198.4	284.8	336.2
Net premiums earned	1,717.0	610.8	700.1
Total realized and unrealized investment gains and net investment income	312.5	278.9	282.5
Loss and loss adjustment expenses incurred, net	1,326.5	465.3	403.5
Acquisition costs, net	387.8	187.1	295.6
Other underwriting expenses	158.8	30.1	34.2
Net premiums written	1,734.2	542.2	657.0
Corporate & Eliminations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred acquisition costs and value of business acquired, net	0.1	0.0	0.0
Loss and loss adjustment expense reserves	894.6	219.7	236.5
Unearned premium	12.5	3.4	2.6
Net premiums earned	(16.7)	28.1	88.6
Total realized and unrealized investment gains and net investment income	317.0	278.9	282.5
Loss and loss adjustment expenses incurred, net	6.3	(0.1)	(4.7)
Acquisition costs, net	(64.6)	25.3	91.0
Other underwriting expenses	24.1	5.9	9.1
Net premiums written	(43.5)	28.9	87.6
Reinsurance | Operating Segments
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred acquisition costs and value of business acquired, net	147.5	69.5	91.9
Loss and loss adjustment expense reserves	3,435.7	1,084.1	872.2
Unearned premium	687.5	261.9	331.9
Net premiums earned	1,210.9	575.6	606.8
Total realized and unrealized investment gains and net investment income	0.3	0.0	0.0
Loss and loss adjustment expenses incurred, net	999.6	459.5	404.3
Acquisition costs, net	302.7	160.4	204.2
Other underwriting expenses	105.5	24.0	24.9
Net premiums written	1,124.9	497.3	563.9
Insurance & Services | Operating Segments
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred acquisition costs and value of business acquired, net	71.2	(0.9)	0.3
Loss and loss adjustment expense reserves	511.1	6.3	3.0
Unearned premium	498.4	19.5	1.7
Net premiums earned	522.8	7.1	4.7
Total realized and unrealized investment gains and net investment income	(4.8)	0.0	0.0
Loss and loss adjustment expenses incurred, net	320.6	5.9	3.9
Acquisition costs, net	149.7	1.4	0.4
Other underwriting expenses	29.2	0.2	0.2
Net premiums written	$ 652.8	$ 16.0	$ 5.5