Summary of Significant Accounting Policies (Basis of Presentation) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2014	Jul. 31, 2012
Basis of Presentation
Net investment income	$ 16.8		$ 13.8	$ 42.6	$ 44.7
Income Tax Expense (Benefit)	$ (1.6)		(4.7)	$ 0.8	11.4
OneBeacon Ltd.
Basis of Presentation
Cumulative Effect on Retained Earnings, Net of Tax						$ 0.9	$ 0.5
Ownership interest (as a percent)	75.30%			75.30%		75.30%
HG Global
Basis of Presentation
Surplus Notes									$ 503.0
Percentage of par value of policy reinsured	15.00%			15.00%
Noncontrolling equity percentage	3.10%			3.10%		3.10%
Star & Shield Insurance Exchange
Basis of Presentation
Surplus Notes	$ 20.0			$ 20.0
Noncontrolling equity percentage	100.00%			100.00%		100.00%
White Mountains Advisors L L C [Member]
Basis of Presentation
Investment Advisory, Management and Administrative Fees	$ 2.0		3.2	$ 5.9	9.2
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%			96.90%
Noncontrolling equity percentage	96.90%			96.90%		96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%			88.40%
Noncontrolling equity percentage	88.40%			88.40%		88.40%
Accounting Standards Update 2014-01 [Member] | Restatement Adjustment [Member] | OneBeacon Ltd.
Basis of Presentation
Net investment income			0.4		1.3
Income Tax Expense (Benefit)			$ 0.6		$ 1.8
OneBeacon Runoff [Member]
Basis of Presentation
Proceeds from Sale of Buildings		$ 58.0
Surplus Notes								$ 80.9