Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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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.

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Amount before accumulated amortization of intangible assets, excluding goodwill.

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Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

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Carrying amount of finite-lived intangible assets, indefinite-lived intangible assets and goodwill. Goodwill is an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Intangible assets are assets, not including financial assets, lacking physical substance.

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Intangible Assets, Gross (Including Goodwill)

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Goodwill and Other Intangible Assets (Goodwill and Intangible Assets) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 10, 2014	Mar. 14, 2014	Feb. 19, 2014
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	$ 213.5	$ 25.0
Amortization of Intangible Assets	16.0	4.3
Intangible Assets, Net (Excluding Goodwill)	197.5	20.7	16.9
Goodwill	168.9us-gaap_Goodwill	0us-gaap_Goodwill	0us-gaap_Goodwill
Intangible Assets, Gross (Including Goodwill)	382.4	25.0
Goodwill and intangible assets	366.4	20.7
Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	146.3	0
Amortization of Intangible Assets	3.5	0
Intangible Assets, Net (Excluding Goodwill)	142.8	0		146.3
Goodwill	145.1	0		145.1
QuoteLab [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	38.5	0
Amortization of Intangible Assets	6.0	0
Intangible Assets, Net (Excluding Goodwill)	32.5	0			38.5
Goodwill	18.3	0			18.3
WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	2.9	0
Amortization of Intangible Assets	0.4	0
Intangible Assets, Net (Excluding Goodwill)	2.5	0
Goodwill	5.5	0				5.5
Developed Technology Rights [Member] \| Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	12.7	0
Amortization of Intangible Assets	0.5	0
Intangible Assets, Net (Excluding Goodwill)	12.2	0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Developed Technology Rights [Member] \| QuoteLab [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	32.0	0
Amortization of Intangible Assets	4.8	0
Intangible Assets, Net (Excluding Goodwill)	27.2	0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Developed Technology Rights [Member] \| WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	0.8	0
Amortization of Intangible Assets	0.2	0
Intangible Assets, Net (Excluding Goodwill)	0.6	0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	3 years
Customer Relationships [Member]
Goodwill [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	3 years
Customer Relationships [Member] \| Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	107.5	0
Amortization of Intangible Assets	2.1	0
Intangible Assets, Net (Excluding Goodwill)	105.4	0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years
Customer Relationships [Member] \| QuoteLab [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	6.5	0
Amortization of Intangible Assets	1.2	0
Intangible Assets, Net (Excluding Goodwill)	5.3	0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	4 years
Customer Relationships [Member] \| Star & Shield Insurance Exchange [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	1.2	0
Amortization of Intangible Assets	0.4	0
Intangible Assets, Net (Excluding Goodwill)	0.8	0
Trademarks [Member] \| WOBI [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	2.1	0
Amortization of Intangible Assets	0.2	0
Intangible Assets, Net (Excluding Goodwill)	1.9	0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	8 years
Other Intangible Assets [Member] \| Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	0.5	0
Amortization of Intangible Assets	0	0
Intangible Assets, Net (Excluding Goodwill)	0.5	0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years
Other Intangible Assets [Member] \| OneBeacon
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	9.4	9.4
Amortization of Intangible Assets	5.7	4.3
Intangible Assets, Net (Excluding Goodwill)	3.7	5.1
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years
Trade Names [Member] \| Tranzact [Member]
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	25.6	0
Amortization of Intangible Assets	0.9	0
Intangible Assets, Net (Excluding Goodwill)	24.7	0
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	6 years
Contract-Based Intangible Assets [Member] \| Sirius Group
Goodwill [Line Items]
Intangible Assets, Gross (Excluding Goodwill)	15.2	15.6
Amortization of Intangible Assets	0	0
Intangible Assets, Net (Excluding Goodwill)	$ 15.2	$ 15.6