Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$ 87.6	$ 85.4
Non-U.S. net operating loss carryforwards	68.2	36.1
Intangible assets	51.5	0.0
Incentive compensation	25.4	21.4
Unearned premiums	18.2	0.0
Accrued interest	10.1	8.8
Loss reserves	8.4	0.0
Deferred acquisition costs	3.7	8.0
Tax credit carryforwards	3.3	1.8
Net unrealized investment losses	0.0	4.7
Other items	1.0	1.3
Total gross deferred tax assets	277.4	167.5
Less: valuation allowances	116.1	94.3
Total net deferred tax assets	161.3	73.2
Deferred tax liabilities related to:
Member’s surplus contributions	82.2	71.3
Purchase accounting	43.9	43.9
Investment basis difference	40.9	33.9
Deferred underwriting	18.3	9.7
Net unrealized investment gains (losses)	15.5	0.0
Other items	1.2	2.2
Total deferred tax liabilities	202.0	161.0
Net deferred tax asset (liability)	$ (40.7)	$ (87.8)