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Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Casualty) (Details) - Casualty - Specialty Products - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 1,411.7
All outstanding liabilities, net of reinsurance	4.3
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	568.7
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	54.0	$ 54.0	$ 53.6	$ 51.9	$ 50.7	$ 44.4	$ 41.7	$ 32.4	$ 24.8	$ 6.8
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	91.5	88.1	85.3	83.1	77.6	65.4	50.8	32.3	$ 8.8
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	187.8	178.3	169.9	161.4	147.8	124.0	81.8	$ 26.5
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	220.3	213.6	196.3	180.5	157.8	107.0	$ 32.0
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	192.5	186.2	163.7	131.3	88.9	$ 25.8
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	219.8	193.6	157.7	86.7	$ 25.8
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	194.2	143.1	86.3	$ 26.4
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	156.7	95.2	$ 34.3
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	78.4	$ 19.9
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 16.5

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Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 4B
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Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 4B
-URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671311-158438

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Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense.

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