Statutory Capital and Surplus (Details) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) yr	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2017	Dec. 31, 2013 USD ($)
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent		$ 3,603.3	$ 3,913.2
Amount available for dividend distributions without prior approval from regulatory authorities		60.0
Amount not available for dividend distribution		502.5
Distributions made		5.4
Fair value investments	$ 3,343.6	5,039.7	4,050.9	$ 3,343.6
Trading Securities		4,256.8
Other long-term investments		323.3	315.8
OneBeacon
Statutory Accounting Practices [Line Items]
Statutory net income (loss)		36.7	44.5	(14.2)
Stockholders' Equity Attributable to Parent		780.6	755.2
Distributions made		79.2	80.0
Unrestricted cash		64.5
Other Operations
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent		2,294.9	1,124.3
Unrestricted cash		1,655.0
Total Capital Contributed to Subsidiary		703.0	676.0
Other long-term investments		66.9
Unregulated Operation
Statutory Accounting Practices [Line Items]
Amount available for dividend distributions without prior approval from regulatory authorities		2,300.0
Unregulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made		5.7	5.3
Unrestricted cash		56.4
White Mountains Advisors LLC | Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash		17.8
Star & Shield Insurance Exchange [Member] | Other Operations
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		4.4	5.0
Statutory net income (loss)		20.4	(3.5)
Surplus required		0.4
OneBeacon
Statutory Accounting Practices [Line Items]
Dividends available, net of noncontrolling interest		177.1
HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Preferred stock, value		$ 619.0
Ownership interest (as a percent)		96.90%
Preferred stock dividend rate		6.00%
HG Re | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		$ 467.2
Collateral Held in Supplement Trust		465.4
BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		431.5
Statutory net income (loss)		32.7	32.0	31.8
Surplus required		66.0
Subsidiaries
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent		562.5
One Beacon Insurance Company | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made	700.5
Noncash and Part Noncash Distribution Received	151.2
Split Rock [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		270.2	249.5
Statutory net income (loss)		$ 42.4	45.5	46.2
Percent Available for Dividend Distribution without Prior Approval from Regulatory Agency in Regulated Entities		15.00%
Split Rock [Member] | Regulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		$ 270.2
Amount available for dividend distributions without prior approval from regulatory authorities		60.1
Distributions made		25.0
Capital		210.1
Statutory accounting practices earned surplus		60.1
Total Capital Contributed to Subsidiary			85.0
Statutory Accounting Practices, Statutory Amount Available for Contributions without Regulatory Approval		31.5
White Mountains | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made		60.3
Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Surplus required		107.5
Atlantic Speciality Insurance Company (ASIC) [Member] | Regulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Amount available for dividend distributions without prior approval from regulatory authorities		11.4
Distributions made		26.5
Capital		624.8
Statutory accounting practices earned surplus		$ 69.0
Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)		8.00%
basis points		3.00%
Preferred stocks | HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends Payable		$ 186.2
Preferred stocks | White Mountains | HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends Payable		180.2
Common Equities [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Trading Securities		11.9
Common Equities [Member] | Other Operations
Statutory Accounting Practices [Line Items]
Trading Securities		285.6
Retained earnings
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	$ 3,010.5	$ 2,797.2	3,084.9	$ 3,010.5		$ 2,801.9
Retained earnings | Split Rock [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory Accounting Practices, Statutory Capital, Percent		25.00%
Retained earnings | Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory Accounting Practices, Statutory Capital, Percent		10.00%
U.S and Bermuda [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		$ 624.8	$ 622.3
Surplus Note | Unregulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Fair value investments		$ 71.9
Combination One | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate		5
Combination Two [Member] | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate | yr		3
US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)		3.54%
Fixed interest rate [Member] | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)		8.00%
Scenario, Forecast [Member] | US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)					3.78%