Loss and Loss Adjustment Expense Reserves (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended												3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 AutoOne		Dec. 31, 2011 AutoOne		Jun. 30, 2011 AutoOne		Mar. 31, 2011 AutoOne		Dec. 31, 2010 AutoOne		Jun. 30, 2012 One Beacon	Jun. 30, 2011 One Beacon	Jun. 30, 2012 One Beacon	Jun. 30, 2011 One Beacon	Jun. 30, 2012 Sirius group	Jun. 30, 2011 Sirius group	Jun. 30, 2012 Sirius group	Jun. 30, 2011 Sirius group
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$ 5,510.8	$ 5,759.8	$ 5,702.3	$ 5,736.8
Less beginning reinsurance recoverable on unpaid losses	(2,465.4)	(2,304.2)	(2,507.3)	(2,344.0)
Net loss and LAE reserves	3,045.4	3,455.6	3,195.0	3,392.8	0	[1]	0	[1]	67.7	[1]	(72.7)	[1]	(77.3)	[1]
Loss and LAE incurred relating to current year losses	268.4	288.7	526.3	664.4
Net favorable and unfavorable loss reserve development	(3.4)	(10.2)	1.4	(27.5)											3.3	10.0	7.2	15.2			5.8	12.3
Total incurred losses and LAE	265.0	278.5	527.7	636.9															112.5	140.6	225.9	368.6
Accretion of fair value adjustment to net loss and LAE reserves	1.2	2.0	8.3	4.1
Foreign currency translation adjustment to net loss and LAE reserves	(12.1)	10.1	(0.3)	31.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	(80.9)	(80.6)	(127.6)	(125.1)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	(258.2)	(211.3)	(642.7)	(481.0)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	(339.1)	(291.9)	(770.3)	(606.1)
Net ending balance	2,960.4	3,449.3	2,960.4	3,449.3	0	[1]	0	[1]	67.7	[1]	(72.7)	[1]	(77.3)	[1]
Plus ending reinsurance recoverable on unpaid losses	2,369.4	2,193.1	2,369.4	2,193.1															328.7		328.7
Gross ending balance	5,329.8	5,642.4	5,329.8	5,642.4
Net favorable loss reserve development due to 2010 Chilean earthquake																						9.0
Net favorable loss reserve development due to 1999 aviation loss																						9.0
Net Unfavorable Loss Reserve Development Due to Asbestos and Environmental Reserves Additions																						2.0
Net Unfavorable Loss Reserve Development Due To Loss From Other Business Lines																						3.7
Accretion of fair value adjustment to net loss and LAE reserves increase			5.0
Remaining unamortized fair value adjustment			$ 3.0

[1] Loss and LAE reserve balances for AutoOne prior to December 31, 2011 were not classified as held for sale for three months ended March 31, 2011. Adjustment is to present loss and LAE reserve activities from continuing operations.