Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets related to:
U.S. federal and state net operating and capital loss carryforwards	$ 79.6	$ 92.3
Non-U.S. net operating loss carryforwards	50.5	41.7
Incentive compensation	17.5	15.0
Accrued interest	7.9	5.1
Deferred acquisition costs	5.5	4.3
Tax credit carryforwards	5.5	4.2
Other items	1.3	3.4
Total gross deferred tax assets	167.8	166.0
Less: valuation allowances	97.4	77.9
Total net deferred tax assets	70.4	88.1
Deferred tax liabilities related to:
Member’s surplus contributions	52.9	43.2
Investment basis difference	11.8	6.6
Purchase accounting	5.1	3.1
Net unrealized investment gains	0.3	66.3
Other items	2.8	1.5
Total deferred tax liabilities	72.9	120.7
Net deferred tax (liability)	$ (2.5)	$ (32.6)