Debt (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 OBH Senior Notes	Dec. 31, 2011 OBH Senior Notes	Sep. 30, 2012 SIG Senior Notes	Dec. 31, 2011 SIG Senior Notes	Sep. 30, 2012 WTM Bank Facility	Dec. 31, 2011 WTM Bank Facility	Aug. 12, 2011 WTM Bank Facility	Sep. 30, 2012 Old Lyme	Dec. 31, 2011 Old Lyme	Dec. 30, 2011 Old Lyme Y	Sep. 30, 2012 Other debt		Dec. 31, 2011 Other debt
Debt Instrument
Debt instrument at face value			$ 269.9	$ 269.9	$ 400.0	$ 400.0
Unamortized original issue discount			(0.1)	(0.1)	(0.6)	(0.7)
Total debt	676.6	677.5	269.8	269.8	399.4	399.3	0	0		2.1	2.1		5.3	[1]	6.3	[1]
Total commitment under revolving credit facility									375.0
Debt Instrument Term (in years)												5
Percentage of principal amount of debt subject to upward or downward adjustments												50.00%
Cash to be paid on acquisition												$ 6.0

[1] (1) Other debt relates to White Mountains’ consolidation of Hamer and Bri-Mar.