Summary of Significant Accounting Policies (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 security Y	Dec. 31, 2010
Significant Accounting Policies
Percentage of the investment portfolio recorded at fair value based upon quoted market prices or other observable inputs	95.00%	93.00%
Maturity period to classify an investment as short-term (in years)	1
Minimum percentage of variation from the expected price required to consider prices provided by the pricing services as outliers	5.00%
Minimum amount of variation from expected prices required to consider prices provided by the pricing services as outliers	$ 1
Number of securities that fell outside White Mountains' variance threshold	4