Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended							3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 UNITED STATES	Dec. 31, 2012 Luxembourg	Dec. 31, 2012 SWEDEN	Dec. 31, 2012 Domestic Tax Authority	Dec. 31, 2012 Sirius Re Holdings Inc.	Dec. 31, 2012 Limited Liability Company	Dec. 31, 2011 Limited Liability Company	Dec. 31, 2010 Limited Liability Company
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon
Fees from Revenue Sharing Agreement with Related Party									$ 0.5	$ 0.2	$ 0.7
2013	0
2014-2018	0			0	0	0
2019-2023	20.2			20.2	0	0
2024-2033	467.0			467.0	0	0
No expiration date	1,743.7			0	1,743.6	0.1
Total	2,230.9			487.2	1,743.6	0.1
Net deferred tax assets
Gross deferred tax asset	674.7			165.2	509.5	0
Valuation allowance	(224.3)			(32.3)	(192.0)	0
Net deferred tax asset	450.4			132.9	317.5	0
Net operating loss carryforwards expected to be utilized					1,308.5
Net operating loss carryforwards, subject to an annual limitation							29.0
Foreign tax credit carryforwards, subject to expiration							10.6	4.6
Valuation allowance	14.1	(128.2)	(2.6)					3.9
Low-income housing tax credit	4.8
Tax credit carryforwards, investments							0.2
Alternative minimum tax credit carryforwards, not subject to expiration	1.5
Deferred tax benefit due to increase in its NOLs			$ 11.9