Catastrophe Reinsurance - Catastrophe Losses and LAE (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ 1,337.9	$ 1,103.2	$ 908.3
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	(20.7)	(20.1)	(57.1)
Catastrophe [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	95.8	60.5	83.8
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	(19.3)	(7.8)	(14.7)
Catastrophe [Member] \| Property and Casualty Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	90.4	56.6	80.7
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	(19.2)	(7.9)	(15.8)
Catastrophe [Member] \| Life and Health Insurance [Member]
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	5.4	3.9	3.1
Catastrophe Loss and LAE Reserves, (Unfavorable) Favorable Development	$ (0.1)	$ 0.1	$ 1.1

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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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