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Common Shareholders’ Equity and Non-controlling Interests (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			1 Months Ended	61 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 31, 2007 quarter Period	Jun. 30, 2022	May 24, 2007
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		1,329,640
Value of common shares repurchased and retired during the period	$ 134.5	$ 79.8	$ 669.1
Value of each common share repurchased and retired during the period (in dollars per share)	$ 617.00	$ 564.00	$ 503.00
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues	28,405	27,310	44,054
Shares issued to directors (in shares)	1,395wtm_StockIssuedDuringPeriodToDirectors	1,590wtm_StockIssuedDuringPeriodToDirectors
Dividends, Common Stock [Abstract]
Cash dividends declared and paid	6.2us-gaap_PaymentsOfDividendsCommonStock	6.2us-gaap_PaymentsOfDividendsCommonStock	6.6us-gaap_PaymentsOfDividendsCommonStock
Dividends delared and paid (in dollars per share)	$ 1.00	$ 1.00	$ 1.00
Non-controlling interests	543.0us-gaap_MinorityInterest	491.9us-gaap_MinorityInterest
Non-controlling Interests
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	0	0	0
Total Non-Controlling Excluding Reciprocals [Member]
Dividends, Common Stock [Abstract]
Non-controlling interests	168.6	65.8
Common Stock
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period			669.1
Dividends, Common Stock [Abstract]
Allocation of fair value of net assets acquired to non-controlling interests			(2.2)
Restricted Stock
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues	23,440	25,720
Performance Shares
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues	3,570
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	217,879	141,535	1,329,640
Combination Three [Member] \| Performance Shares
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues			9,577
Combination One [Member] \| Restricted Stock
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues			32,160
Combination Two [Member]
Common Shares Issued [Abstract]
Shares issued to directors (in shares)			700
Combination Four [Member]
Common Shares Issued [Abstract]
Shares issued to directors (in shares)			1,617
General Board Authorization [Member]
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares	338,092
Common shares repurchased and retired during the period	207,404	140,000	502,801
Value of common shares repurchased and retired during the period	128.2	78.9	256.0
Value of each common share repurchased and retired during the period (in dollars per share)	$ 618.00	$ 564.00	$ 508.00
Repurchased for Employee Benefit Plans [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	10,475
Repurchased for Employee Benefit Plans [Member] \| Combination Three [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		1,535	10,010
Tender Offer [Member] \| Combination Two [Member]
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period			408.6
Value of each common share repurchased and retired during the period (in dollars per share)			$ 500.00
Tender Offer [Member] \| Combination Two [Member] \| Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period			816,829
White Mountains
Dividends, Common Stock [Abstract]
Capital contributions		594.5	594.5
Sirius Internation Group Ltd \| Preferred stocks
Dividends, Common Stock [Abstract]
Non-cumulative perpetual preference shares issued				250,000,000
Preferred Stock, Liquidation Preference Per Share						$ 1,000
Proceeds from Preference Shares				245.7
Payments of Stock Issuance Costs				4.3
Right to elect directors on board number				2
Number of Quarterly Period for Non Payment of Dividend which Give Right to Elect Directors on Board				6
Minimum Required Payment Period for Dividend to Cease Right to Elect Directors on Board				4
Preference shares floating rate of dividend amount in excess of libor				3.20%
Preferred Stock Redemption Basis Spread on Treasury Issue Rate				45.00%
OneBeacon Ltd.
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	24.70%	24.80%
Non-controlling interests	258.7	273.7
HG/BAM [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	100.00%	100.00%
Non-controlling interests	(121.9)	(97.6)
Star & Shield Insurance Exchange [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	100.00%	0.00%
Non-controlling interests	(12.4)	0
HG Global
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	3.10%	2.70%
Non-controlling interests	17.9	16.6
QuoteLab [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	40.00%	0.00%
Non-controlling interests	22.6	0
Tranzact [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	36.80%	0.00%
Non-controlling interests	88.2	0
WOBI [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	36.70%	0.00%
Non-controlling interests	5.4	0
Prospector Offshore Fund Ltd [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	23.40%	28.30%
Non-controlling interests	31.1	32.1
Prospector Turtle Fund [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	0.00%	31.10%
Non-controlling interests	0	14.1
A W G Dewar Inc [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	18.00%	18.00%
Non-controlling interests	3.4	3.0
SIG Preference Shares
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	100.00%	100.00%
Non-controlling interests	250.0	250.0
BAM Management
Dividends, Common Stock [Abstract]
Capital contributions		14.5	14.5
BAM Management \| Non-controlling Interests
Dividends, Common Stock [Abstract]
Allocation of fair value of net assets acquired to non-controlling interests			2.2
Interest Rate Cap \| Sirius Internation Group Ltd
Dividends, Common Stock [Abstract]
years of interest rate cap	5 years	5 years
Derivative, Cap Interest Rate	8.30%	8.30%
Interest Rate Cap \| Sirius Internation Group Ltd \| Preferred stocks
Dividends, Common Stock [Abstract]
Non-cumulative perpetual preference shares issued				250,000,000
Preference shares reference rate for dividend				3
Preferred stock dividend rate				7.506%
OneBeacon
Dividends, Common Stock [Abstract]
Non-controlling interests	$ 262.1	$ 276.8
OneBeacon \| Scenario, Forecast [Member]
Dividends, Common Stock [Abstract]
years of interest rate cap					5 years