Debt (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 OBH Senior Notes	Dec. 31, 2011 OBH Senior Notes	Jun. 30, 2012 SIG Senior Notes	Dec. 31, 2011 SIG Senior Notes	Jun. 30, 2012 Old Lyme	Dec. 31, 2011 Old Lyme	Dec. 30, 2011 Old Lyme Y	Jun. 30, 2012 WTM Bank Facility	Dec. 31, 2011 WTM Bank Facility	Aug. 12, 2011 WTM Bank Facility	Jun. 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	677.6	677.5	269.8	269.8	399.4	399.3	2.1	2.1		0	0		6.3	[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%
Business Acquisition, Cost of Acquired Entity, Cash Paid									$ 6.0

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