Catastrophe Reinsurance - Catastrophe Losses and LAE (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ (2,086.0)	$ (1,473.0)	$ (1,337.9)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	7.4	(18.9)	20.7
Catastrophe [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(87.6)	(180.4)	(95.8)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	8.4	4.5	19.3
Catastrophe [Member] \| Property and Casualty Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(79.1)	(168.8)	(84.1)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	8.2	4.7	19.0
Catastrophe [Member] \| Specialty Property & Casualty Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(4.4)	(5.2)	(6.3)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	0.3	0.3	0.2
Catastrophe [Member] \| Life and Health Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(4.1)	(6.4)	(5.4)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	(0.1)	(0.5)	$ 0.1
Reinsurance Recoverable FHCF [Member] \| Catastrophe [Member] \| Property and Casualty Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(31.8)	(11.9)
Reinsurance Recoverable FHCF [Member] \| Catastrophe [Member] \| Life and Health Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ (1.6)	$ (0.2)

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No definition available.

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- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437

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- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437

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