Income Taxes - Effective Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract]
U.S. federal statutory tax rate	$ (279.0)	$ (66.5)	$ (118.7)
State income taxes, net of federal income tax effect	(11.1)	2.2	(5.8)
Effects of changes in tax laws or rates enacted in current period	0.0	0.0	0.0
Withholding tax	(2.4)	(1.0)	(2.3)
Branch profits tax	(3.9)	(2.7)	(5.0)
Other	0.0	(0.3)	0.0
Foreign tax credit	0.0	0.0	1.6
Changes in valuation allowances	(9.9)	(12.3)	(1.4)
Members surplus contribution	0.0	(2.8)	(5.7)
Noncontrolling interest	6.4	3.9	6.8
Non-taxable foreign income	16.0	1.4	2.5
Other	(6.6)	(4.0)	1.8
Changes in unrecognized tax benefits	0.0	0.0	0.0
Total income tax (expense) benefit	$ (126.9)	$ (32.6)	$ 15.5
Effective Income Tax Rate Reconciliation, Percent [Abstract]
U.S. federal statutory tax rate	21.00%	21.00%	21.00%
State income taxes, net of federal income tax effect	0.80%	(0.70%)	1.00%
Effects of changes in tax laws or rates enacted in current period	0.00%	0.00%	0.00%
Withholding tax	0.20%	0.30%	0.40%
Branch profits tax	0.30%	0.80%	0.90%
Other	0.00%	0.10%	0.00%
Foreign tax credit	0.00%	0.00%	(0.30%)
Changes in valuation allowances	0.80%	3.90%	0.30%
Members surplus contribution	0.00%	0.90%	1.00%
Noncontrolling interest	(0.50%)	(1.20%)	(1.20%)
Non-taxable foreign income	(1.20%)	(0.50%)	(0.40%)
Other	0.50%	1.30%	(0.30%)
Changes in unrecognized tax benefits	0.00%	0.00%	0.00%
Effective rate	9.60%	10.30%	(2.70%)
United Kingdom
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory rate difference	$ (3.2)	$ (2.6)	$ (2.0)
Other	$ (1.0)	$ 2.9	$ (0.2)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory tax rate difference between U.K. and U.S.	0.20%	0.80%	0.40%
Other	0.10%	(0.90%)	0.00%
LUXEMBOURG
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract]
Changes in valuation allowances	$ 6.6	$ 4.2	$ (31.6)
Other	(0.1)	(2.3)	(0.2)
Statutory rate difference	(25.3)	(0.3)	(1.9)
Participation exemption	207.9	0.0	8.5
Write-down of investment	$ 0.0	$ 0.0	$ 35.4
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Changes in valuation allowances	(0.50%)	(1.30%)	5.60%
Statutory tax rate difference between U.K. and U.S.	1.90%	0.10%	0.30%
Other	0.00%	0.70%	0.00%
Participation exemption	(15.60%)	0.00%	(1.50%)
Write-down of investment	0.00%	0.00%	(6.30%)
Bermuda
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract]
Effects of changes in tax laws or rates enacted in current period	$ 0.0	$ 5.0	$ 68.0
Other	0.0	0.0	(0.1)
Statutory rate difference	53.2	44.1	63.4
Write-down of deferred tax asset	$ (73.0)	$ 0.0	$ 0.0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effects of changes in tax laws or rates enacted in current period	0.00%	(1.60%)	(12.00%)
Other	0.00%	0.00%	0.00%
Statutory tax rate difference between U.K. and U.S.	(4.00%)	(13.90%)	(11.20%)
Write-down	5.50%	0.00%	0.00%
Other foreign
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory rate difference	$ (1.5)	$ (1.5)	$ 2.4
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory tax rate difference between U.K. and U.S.	0.10%	0.50%	(0.40%)