Investments - Net Investment Losses (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Gain (Loss) on Securities
Net investment gains (losses)	$ (5.9)	$ (17.4)	$ (3.7)	$ (21.3)
Fixed maturity securities
Gain (Loss) on Securities
Net investment gains (losses)	(2.3)	(14.7)	(3.3)	(17.1)
Equity securities
Gain (Loss) on Securities
Net investment gains (losses)	(2.1)	(3.5)	0.5	(4.5)
Short-term investments and other
Gain (Loss) on Securities
Net investment gains (losses)	$ (1.5)	$ 0.8	$ (0.9)	$ 0.3

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(9)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(7)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-9

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