Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$ 102.8	$ 79.6
Non-U.S. net operating loss carryforwards	47.1	50.5
Incentive compensation	14.6	17.5
Accrued interest	10.8	7.9
Deferred acquisition costs	6.4	5.5
Tax credit carryforwards	4.9	5.5
Other items	0.7	1.3
Total gross deferred tax assets	187.3	167.8
Less: valuation allowances	92.3	97.4
Total net deferred tax assets	95.0	70.4
Deferred tax liabilities related to:
Member’s surplus contributions	60.4	52.9
Purchase accounting	48.5	5.1
Investment basis difference	33.7	11.8
Net unrealized investment gains (losses)	10.7	0.3
Deferred underwriting	4.1	0.0
Other items	2.7	2.8
Total deferred tax liabilities	160.1	72.9
Net deferred tax asset (liability)	$ (65.1)	$ (2.5)