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Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Development) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Premiums earned
Current Accident Year		$ 1,175.3	$ 1,088.9	$ 1,235.8
Prior Accident Year		(6.0)	(48.4)	(41.9)
Total incurred losses and LAE		1,169.3	1,040.5	1,193.9
OneBeacon
Premiums earned
Current Accident Year	33.7	33.7
Prior Accident Year	14.3	75.5		(7.4)
Total incurred losses and LAE		109.2	89.8
Professional Insurance \| OneBeacon
Premiums earned
Current Accident Year		22.9
Prior Accident Year		46.4
Total incurred losses and LAE		69.3	59.1
Specialty Property \| OneBeacon
Premiums earned
Current Accident Year		(1.1)
Prior Accident Year		5.7
Total incurred losses and LAE		4.6	1.1
Crop \| OneBeacon
Premiums earned
Current Accident Year		3.8
Prior Accident Year		0
Total incurred losses and LAE		3.8	0
Other \| OneBeacon
Premiums earned
Current Accident Year		2.8
Prior Accident Year		(0.4)
Total incurred losses and LAE		2.4	1.6
Specialty Products \| OneBeacon
Premiums earned
Current Accident Year		28.4
Prior Accident Year		51.7
Total incurred losses and LAE		80.1	61.8
Entertainment \| OneBeacon
Premiums earned
Current Accident Year		1.5
Prior Accident Year		11.6
Total incurred losses and LAE		13.1	13.5
Accident \| OneBeacon
Premiums earned
Current Accident Year		0
Prior Accident Year		3.5
Total incurred losses and LAE		3.5	6.0
Government Risks \| OneBeacon
Premiums earned
Current Accident Year		1.2
Prior Accident Year		7.1
Total incurred losses and LAE		8.3	8.5
Other \| OneBeacon
Premiums earned
Current Accident Year		2.6
Prior Accident Year		1.6
Total incurred losses and LAE		4.2	0
Specialty Industries \| OneBeacon
Premiums earned
Current Accident Year		5.3
Prior Accident Year		23.8
Total incurred losses and LAE		$ 29.1	$ 28.0