Catastrophe Reinsurance - Catastrophe Losses and LAE (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ (1,473.0)	$ (1,337.9)	$ (1,103.2)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	(18.9)	20.7	20.1
Catastrophe [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(180.4)	(95.8)	(60.5)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	4.5	19.3	7.8
Catastrophe [Member] \| Property and Casualty Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(174.0)	(90.4)	(56.6)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	5.0	19.2	7.9
Catastrophe [Member] \| Life and Health Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(6.4)	(5.4)	(3.9)
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	(0.5)	$ 0.1	$ (0.1)
Reinsurance Recoverable [Member] \| Catastrophe [Member] \| Property and Casualty Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	(11.9)
Reinsurance Recoverable FHCF [Member] \| Catastrophe [Member] \| Life and Health Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ (0.2)

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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

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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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-18)
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915

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