Income Taxes (Effective Rate Reconciliation) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended									0 Months Ended	12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 SWEDEN	Dec. 31, 2011 SWEDEN	Dec. 31, 2010 SWEDEN	Dec. 31, 2012 Luxembourg	Dec. 31, 2011 Luxembourg	Dec. 31, 2010 Luxembourg	Jan. 02, 2013 Sirius Group SWEDEN	Dec. 31, 2012 Sirius Group SWEDEN
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)	35.00%
Effective income tax																		22.00%	26.30%
Increase (Decrease) in Deferred Income Taxes																		$ 65.4
Tax (expense) benefit at the U.S. statutory rate									(92.0)	(34.3)	(66.2)
Differences in taxes resulting from:
Tax rate change enacted												65.4	0	0	7.2	1.2	2.8
Non-U.S. earnings, net of foreign taxes									43.0	6.2	22.8
Change in valuation allowance									(14.1)	128.2	2.6
Purchase of subsidiaries									5.1	0	4.5
Tax exempt interest and dividends									(1.3)	4.3	(2.8)
Withholding tax									(2.9)	0.2	(0.2)
Tax reserve adjustments									3.1	2.9	2.3
Sale of subsidiaries									0	0	4.2
Other, net									2.2	1.3	0.4
Total income tax benefit (expense)	101.0	(47.8)	(6.4)	(31.1)	110.9	0.6	(8.8)	7.3	15.7	110.0	(29.6)
Non-U.S. component of pre-tax income									250.0	65.4	74.4
Tax Payments and Receipts
Net income tax payments to (receipts from) national governments, total									$ 17.5	$ 12.5	$ (47.0)