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Significant Transactions (Dispositions) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended	0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Dec. 23, 2014	Oct. 31, 2014
Significant Transactions
(Loss) gain on sale of discontinued operations, net of tax	$ (1.6)	$ 46.6	$ (91.0)
Gain (Loss) on Disposition of Stock in Subsidiary	0.7	21.3	0
Premiums Written, Net	2,121.5us-gaap_PremiumsWrittenNet
OneBeacon Runoff
Significant Transactions
Surplus notes					101.0
Ashmere [Member] \| WM Solutions
Significant Transactions
Excess amount over statutory surplus						6.0
Gain (Loss) on Disposition of Stock in Subsidiary	1.0
Citation [Member] \| WM Solutions
Significant Transactions
Gain (Loss) on Disposition of Stock in Subsidiary				0.7
Essentia \| OneBeacon
Significant Transactions
Gain (Loss) on Disposition of Stock in Subsidiary		23.0
Gain (Loss) on Disposition of Stock in Subsidiary, After Tax		15.0
Underwriting Agreement [Member] \| Essentia \| OneBeacon
Significant Transactions
Premiums Written, Net			179.7
Premiums Written, Percentage of Total			8.00%
Fair Value [Member] \| Other long-term investments \| OneBeacon Runoff
Significant Transactions
Surplus notes					64.9
Discontinued Operations
Significant Transactions
(Loss) gain on sale of discontinued operations, net of tax	(1.6)	46.6	(91.0)
Discontinued Operations \| OneBeacon Runoff
Significant Transactions
Surplus notes	101.0
(Loss) gain on sale of discontinued operations, net of tax	(35.9)		91.5
Discontinued Operations \| Surplus Note \| OneBeacon Runoff
Significant Transactions
(Loss) gain on sale of discontinued operations, net of tax	(23.5)
Discontinued Operations \| Fair Value [Member] \| OneBeacon Runoff
Significant Transactions
Surplus notes	65.1
Surplus Note \| Discontinued Operations \| OneBeacon Runoff
Significant Transactions
(Loss) gain on sale of discontinued operations, net of tax	$ 36.1				$ 36.1