Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	May. 31, 2007	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period		$ 284.2	$ 134.5	$ 79.8
Value of each common share repurchased and retired during the period (in dollars per share)		$ 733	$ 617	$ 564
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		25,016	28,405	27,310
Shares issued to directors (in shares)		1,376	1,395	1,590
Dividends, Common Stock [Abstract]
Cash dividends declared and paid		$ 6.0	$ 6.2	$ 6.2
Dividends delared and paid (in dollars per share)		$ 1.00	$ 1.00	$ 1.00
Non-controlling interests		$ 454.8	$ 542.7
Non-controlling Interests
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period		0.0	0.0	$ 0.0
Total Non-Controlling Excluding Reciprocals [Member]
Dividends, Common Stock [Abstract]
Non-controlling interests		$ 115.2	$ 168.6
Restricted Stock
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		23,640	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		387,495	217,879	141,535
General Board Authorization [Member]
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares		476,253
Common shares repurchased and retired during the period		361,839	207,404	140,000
Value of common shares repurchased and retired during the period		$ 267.4	$ 128.2	$ 78.9
Value of each common share repurchased and retired during the period (in dollars per share)		$ 739	$ 618	$ 564
Repurchased for Employee Benefit Plans [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		10,802		1,535
Repurchased for Employee Benefit Plans [Member] | Combination Three [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period			10,475
White Mountains
Dividends, Common Stock [Abstract]
Capital contributions					$ 594.5
Sirius Internation Group Ltd | Preferred stocks
Dividends, Common Stock [Abstract]
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		24.50%	24.70%
Non-controlling interests		$ 245.6	$ 258.4
HG/BAM [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ (140.0)	$ (121.9)
Star & Shield Insurance Exchange [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ (16.0)	$ (12.4)
HG Global
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		3.10%	3.10%
Non-controlling interests		$ 17.1	$ 17.9
MediaAlpha
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		40.00%	40.00%
Non-controlling interests		$ 14.4	$ 22.6
Tranzact [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		36.80%	36.80%
Non-controlling interests		$ 79.4	$ 88.2
WOBI [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		3.90%	36.70%
Non-controlling interests		$ 0.6	$ 5.4
Prospector Offshore Fund Ltd [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		0.00%	23.40%
Non-controlling interests		$ 0.0	$ 31.1
A W G Dewar Inc [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		19.00%	18.00%
Non-controlling interests		$ 3.7	$ 3.4
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
OneBeacon
Dividends, Common Stock [Abstract]
Non-controlling interests		$ 249.3	$ 261.8