Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$ 85.4	$ 85.7
Non-U.S. net operating loss carryforwards	36.1	46.1
Incentive compensation	21.4	14.5
Accrued interest	8.8	7.8
Deferred acquisition costs	8.0	6.4
Net unrealized investment losses	4.7	0.0
Tax credit carryforwards	1.8	0.2
Other items	0.5	0.4
Total gross deferred tax assets	166.7	161.1
Less: valuation allowances	94.3	85.6
Total net deferred tax assets	72.4	75.5
Deferred tax liabilities related to:
Member’s surplus contributions	71.3	60.4
Purchase accounting	43.9	43.9
Investment basis difference	33.9	26.8
Deferred underwriting	9.7	4.1
Net unrealized investment gains (losses)	0.0	10.7
Other items	1.4	1.8
Total deferred tax liabilities	160.2	147.7
Net deferred tax asset (liability)	$ (87.8)	$ (72.2)