Goodwill and Intangible Assets - Schedule of Goodwill by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Goodwill [Roll Forward]
Goodwill	$ 1,312.0	$ 1,114.0
Specialty Property & Casualty Insurance
Goodwill [Roll Forward]
Goodwill	1,043.0	845.0
Preferred Property & Casualty Insurance
Goodwill [Roll Forward]
Goodwill	49.6	49.6
Life & Health Insurance
Goodwill [Roll Forward]
Goodwill	$ 219.4	$ 219.4

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
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