Significant Transactions (Formation of HG and BAM) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2012
Significant Transactions
Percentage of par value of policy reinsured		15.00%
White Mountains
Significant Transactions
Funds to capitalize HG Global	$ 600.0	$ 594.5
HG Global
Significant Transactions
Ownership interest (as a percent)		97.30%
Surplus notes		503.0
Statutory capital and surplus		412.0
Percentage of par value of policy reinsured		15.00%
Income (loss) from continuing operations		14.2
Interest income (expense), surplus notes		18.4
HG Global | Common shareholders’ equity
Significant Transactions
Ownership interest (as a percent)		88.70%
HG Global | Redeemable preferred stocks
Significant Transactions
Ownership interest (as a percent)		97.30%
Build America Mutual
Significant Transactions
Income (loss) from continuing operations		(36.3)
Interest income (expense), surplus notes		(18.4)
HG Global
Significant Transactions
Statutory capital and surplus		$ 412.0
Percentage of par value of policy reinsured		15.00%