Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended			6 Months Ended	0 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2013	Dec. 23, 2014	Dec. 31, 2011
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									$ (1.6)	$ 46.6	$ (91.0)
Loss and loss adjustment expense reserves	3,159.8				3,079.3				3,159.8	3,079.3	3,168.9			5,702.3
Loss and loss adjustment expenses									1,169.3	1,040.5	1,193.9
Other revenue									62.4	57.5	100.3
Net loss from discontinued operations, net of tax	(12.2)	6.7	2.6	(0.5)	(0.3)	0.4	3.9	0.5	(1.8)	(42.1)	(24.0)
Reinsurance recoverable on paid losses	23.6				25.4				23.6	25.4
Prior Accident Year									(6.0)	(48.4)	(41.9)
Discontinued Operations
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									(1.6)	46.6	(91.0)
Loss and loss adjustment expense reserves					1,793.1					1,793.1
Net (loss) income from discontinued operations									(3.4)	4.5	(115.0)
Loss and loss adjustment expenses									(0.7)	78.9	48.4
Other revenue									0	10.8	0
Net loss from discontinued operations, net of tax									(1.8)	(42.1)	(24.0)
Reinsurance recoverable on paid losses					10.7					10.7
Runoff Entities [Member] | Discontinued Operations
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									(18.8)
Esurance [Member] | White Mountains
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									3.2
OneBeacon Runoff
Discontinued Operations
Surplus notes													101.0
OneBeacon Runoff | Discontinued Operations
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									(35.9)		91.5
Transfer of assets	7.4								7.4
Surplus notes	101.0								101.0
Net loss from discontinued operations, net of tax											24.0
Prior Accident Year										(71.5)
liability for unpaid claims and claims adjustment expenses, incurred claims, prior years, after-tax										46.5
AutoOne | Discontinued Operations
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax											0.5
Fireman's Fund Insurance Company [Member] | White Mountains
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									13.9
Surplus Note | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									(23.5)
Reserve Charge [Member] | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									4.8
Reserve charge under the SPA, before tax												7.4
Fair Value [Member] | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Surplus notes	65.1								65.1
Fair Value [Member] | Other long-term investments | OneBeacon Runoff
Discontinued Operations
Surplus notes													64.9
Safeco v. American International Group, Inc. [Member] | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Other revenue							7.8
Surplus Note | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
(Loss) gain on sale of discontinued operations, net of tax									36.1				36.1
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax									$ 23.5