Third Party Reinsurance (Sirius Group) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Reinsurance [Line Items]
Reinsurance recoverable on paid losses	$ 16.1			$ 17.9
Reinsurance recoverable on unpaid losses	388.6		410.8	429.1	2,369.4	2,465.4	2,507.3
Sirius
Reinsurance [Line Items]
Reinsurance recoverable on paid losses	13.3
Reinsurance recoverable on unpaid losses	306.8
Berkshire Hathaway [Member] | Sirius
Reinsurance [Line Items]
Reinsurance recoverables	56.3
Percentage of total paid and unpaid reinsurance recoverables	18.00%
Percentage Collateralized	0.00%
Swiss Re Group | Sirius
Reinsurance [Line Items]
Reinsurance recoverables	34.0
Percentage of total paid and unpaid reinsurance recoverables	11.00%
Percentage Collateralized	0.00%
Olympus | Sirius
Reinsurance [Line Items]
Reinsurance recoverables	25.0	[1]
Percentage of total paid and unpaid reinsurance recoverables	8.00%	[1]
Percentage Collateralized	100.00%	[1]
Lloyds of London | Sirius
Reinsurance [Line Items]
Reinsurance recoverables	16.1	[2]
Percentage of total paid and unpaid reinsurance recoverables	5.00%	[2]
Percentage Collateralized	16.00%	[2]
International Medical Insurance Co Ltd [Member] | Sirius
Reinsurance [Line Items]
Reinsurance recoverables	$ 12.2
Percentage of total paid and unpaid reinsurance recoverables	4.00%
Michigan Catastorphic Claims Association [Member] | Sirius
Reinsurance [Line Items]
Percentage Collateralized	31.00%

[1] Non-U.S. insurance entity. The balance is fully collateralized through trust agreements or funds held.

[2] Represents the total of reinsurance recoverables due to Sirius Group from all Lloyds Syndicates.