Third Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - All Entities Except TCP Providers $ in Millions	Sep. 30, 2021 USD ($)
Reinsurance [Line Items]
Reinsurance recoverables	$ 165.7
Reinsurance Recoverables, Collateral	48.3
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 117.4
Percent of reinsurance recoverables from insurers that are collateralized	100.00%
A+ or better
Reinsurance [Line Items]
Reinsurance recoverables	$ 122.8
Reinsurance Recoverables, Collateral	23.9
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 98.9
Percent of reinsurance recoverables from insurers that are collateralized	84.20%
A- to A
Reinsurance [Line Items]
Reinsurance recoverables	$ 32.7
Reinsurance Recoverables, Collateral	15.6
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 17.1
Percent of reinsurance recoverables from insurers that are collateralized	14.60%
B++ or lower and not rated
Reinsurance [Line Items]
Reinsurance recoverables	$ 10.2
Reinsurance Recoverables, Collateral	8.8
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 1.4
Percent of reinsurance recoverables from insurers that are collateralized	1.20%