Discontinued Operations (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended			51 Months Ended
	Dec. 23, 2014	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2012
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax										$ 18.2	$ (1.6)	$ 46.6
Net realized and unrealized investment gains										225.4	78.5	133.9
Net investment income										60.8	59.5	59.8
Loss and loss adjustment expense reserves		$ 1,395.8				$ 1,350.0				1,395.8	1,350.0	1,054.3	$ 1,395.8	$ 1,000.0
Loss and loss adjustment expenses										708.9	824.0	622.1
Other revenue										334.2	131.1	47.0
Net income from discontinued operations, net of tax		7.3	$ 14.2	$ 3.5	$ 73.8	82.1	$ 63.9	$ 61.1	$ 51.5	80.6	260.2	99.9
Reinsurance recoverable on paid losses		7.5				12.2				7.5	12.2		7.5
Prior Accident Year										(4.0)	92.0	0.0
Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax												46.6
Net realized and unrealized investment gains										15.1	205.4	27.8
Net investment income										40.7	40.9	46.7
Total income from discontinued operations										98.8	258.6	146.5
Loss and loss adjustment expenses										422.7	344.6	497.3
Other revenue										(21.3)	(62.4)	27.6
Net income from discontinued operations, net of tax										80.6	260.2	99.9
Esurance [Member] | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax										17.9	3.2		28.3
Net income from discontinued operations, net of tax												4.5
OneBeacon Runoff
Discontinued Operations
Surplus notes	$ 101.0
OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax										0.3	(18.8)	46.6
Sirius Group
Discontinued Operations
Net realized and unrealized investment gains										225.0	4.0	1.0
Net investment income										225.0	4.0	1.0
Sirius Group | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax												0.0
Net realized and unrealized investment gains										15.1	205.4	27.8
Net investment income										40.7	40.9	46.7
Loss and loss adjustment expense reserves		1,644.4				1,809.8				1,644.4	1,809.8		1,644.4
Total income from discontinued operations										81.1	262.0	142.0
Loss and loss adjustment expenses										422.7	345.3	418.4
Other revenue										(20.6)	(62.4)	16.8
Net income from discontinued operations, net of tax										81.1	262.0	$ 142.0
Reinsurance recoverable on paid losses		$ 10.2				11.4				$ 10.2	11.4		$ 10.2
Fireman's Fund Insurance Company [Member] | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax											14.0
Surplus Note
Discontinued Operations
RBC Score										250.00%
Surplus Note | OneBeacon Runoff
Discontinued Operations
RBC Score	250.00%
Fair Value [Member] | Other long-term investments | OneBeacon Runoff
Discontinued Operations
Surplus notes	$ 64.9
Surplus Note | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax						36.1
Gain (loss) on sale of discontinued operations, net of tax	36.1
Assets, Fair Value Adjustment, Net of Tax	23.5
Interest rate (as a percent)		6.00%								6.00%			6.00%
RBC Score										200.00%
Symetra | Sirius Group
Discontinued Operations
Net realized and unrealized investment gains										$ 218.5
OneBeacon
Discontinued Operations
Gain (loss) on sale of discontinued operations, net of tax	(36.1)									(49.5)	(35.9)
Par Value [Member] | Surplus Note | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Surplus notes	101.0
Par Value [Member] | OneBeacon
Discontinued Operations
Surplus notes	101.0	$ 101.0				101.0				101.0	101.0		$ 101.0
Fair Value [Member] | Surplus Note | OneBeacon Runoff | Discontinued Operations
Discontinued Operations
Surplus notes	64.9
Fair Value [Member] | OneBeacon
Discontinued Operations
Surplus notes	$ 64.9	$ 51.5				$ 65.1				$ 51.5	$ 65.1		$ 51.5