Reinsurance and Catastrophes - Total Amounts of Reinsurance Recoverables On Unpaid Insurance Reserves (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Summary of reinsurance recoverable on unpaid insurance reserves
Reinsurance balances receivable	$ 424.2	$ 462.5
Reinsurance companies
Summary of reinsurance recoverable on unpaid insurance reserves
Reinsurance balances receivable	7.4	3.1
State insurance facilities
Summary of reinsurance recoverable on unpaid insurance reserves
Reinsurance balances receivable	96.6	97.7
Group benefits
Summary of reinsurance recoverable on unpaid insurance reserves
Reinsurance balances receivable	306.2	352.4
Life and health
Summary of reinsurance recoverable on unpaid insurance reserves
Reinsurance balances receivable	$ 14.0	$ 9.3

Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479838/944-20-50-5

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 45
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479616/944-310-45-5

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(6))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1

Name:	us-gaap_ReinsuranceRecoverables
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant