Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended				9 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2014
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	$ (6.0)	$ (48.4)	$ (41.9)
Increase (decrease) in reserves	1,169.3	1,040.5	1,193.9
Current Accident Year	1,175.3	1,088.9	1,235.8
Accretion of fair value adjustment to net loss and LAE reserves	0.7	1.7	10.6
Un-accreted fair value, adjustment to loss and loss adjustment expense reserves, before tax	4.0
Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	(98.0)	(23.9)	(34.5)
Liability for asbestos and environmental claims, gross	238.5	227.8	228.6	207.2
Liability for asbestos claims, net, period increase (decrease)	8.0	11.8	33.0	46.4
Liability for asbestos and environmental claims	210.2	193.9	189.4	162.7
Liability for Asbestos Claims, Net, Period Increase (Decrease) Based on Quarterly Reporting			14.0
Liabilty for environmental claims, net, period increase (decrease)	1.6	0.8	0.5
Asbestos and Environmental Loss Three Year Survival Ratio Period	9 years	8 years
OneBeacon
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	75.5		(7.4)		14.3
Increase (decrease) in reserves	109.2	89.8
Current Accident Year	33.7				33.7
Liability for asbestos and environmental claims, gross	1.0
Star & Shield Insurance Exchange [Member]
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	2.2
Asbestos | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Liability for asbestos and environmental claims, gross	215.8	207.4	208.2	185.1
Liability for asbestos and environmental claims	192.8	178.7	174.2	146.2
Environmental | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Liability for asbestos and environmental claims, gross	22.7	20.4	20.4	22.1
Liability for asbestos and environmental claims	17.4	15.2	15.2	16.5
WM Solutions | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Liability for asbestos claims, net, period increase (decrease)	22.9	13.0	11.0
Liabilty for environmental claims, net, period increase (decrease)	2.1	1.0	0.7
Property [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	(54.0)	(16.6)	(28.4)
Professional Insurance | OneBeacon
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	46.4
Increase (decrease) in reserves	69.3	59.1
Current Accident Year	22.9
Property, Liability and Casualty Insurance Product Line | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	(13.0)
asbestos and environmental [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	10.0
Property Loss Held Above Actuarial Central Estimate [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	(24.0)
Aviation Space [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	13.0	(10.1)	(5.3)
Accident and Health Insurance Product Line [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	(13.0)	(9.2)
Crop | OneBeacon
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	0
Increase (decrease) in reserves	3.8	0
Current Accident Year	3.8
Specialty Products | OneBeacon
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	51.7
Increase (decrease) in reserves	80.1	61.8
Current Accident Year	28.4
Asbestos [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		11.8	46.4
Casualty Runoff [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year			(31.5)
Marine Energy [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year			(11.6)
Trade Credit [Member] | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year			(6.8)
Accident and Health | Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year			4.3
Management's estimate [Member] | Specialty Products | OneBeacon
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Current Accident Year	$ 29.9