Significant Transactions (Formation of HG and BAM) (Details) (USD $) In Millions, unless otherwise specified		12 Months Ended	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 White Mountains	Sep. 30, 2012 HG Global	Dec. 31, 2013 HG Global	Dec. 31, 2012 HG Global	Dec. 31, 2013 HG Global Total	Dec. 31, 2012 HG Global Total	Dec. 31, 2013 HG Global Redeemable preferred stocks	Dec. 31, 2012 HG Global Redeemable preferred stocks	Dec. 31, 2013 HG Re	Dec. 31, 2012 HG Re	Dec. 31, 2013 HG Global
Significant Transactions
Capital contributions		$ 594.5	$ 600.0
Ownership interest (as a percent)				97.30%		88.70%	88.70%	97.30%	97.30%
Surplus notes				503.0	503.0
Statutory capital and surplus										$ 436.9	$ 412.0
Percentage of par value of policy reinsured	15.00%			15.00%								15.00%