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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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period		$ 887.2	$ 284.2	$ 134.5
Value of each common share repurchased and retired during the period (in dollars per share)		$ 802	$ 733	$ 617
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		47,030	25,016	28,405
Shares issued to directors (in shares)		1,735	1,376	1,395
Dividends, Common Stock [Abstract]
Cash dividends declared and paid		$ 5.4	$ 6.0	$ 6.2
Dividends delared and paid (in dollars per share)		$ 1.00	$ 1.00	$ 1.00
Non-controlling interests		$ 133.9	$ 454.8
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		$ 35.8	$ 115.2
Restricted Stock
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		25,365	23,640	23,440
Performance Shares
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		5,000		3,570
Common Stock
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		1,106,145	387,495	217,879
General Board Authorization [Member]
Common Shares Repurchased and Retired [Abstract]
Additional authorized repurchase of common shares		878,130
Common shares repurchased and retired during the period		1,098,123	361,839	207,404
Value of common shares repurchased and retired during the period		$ 881.4	$ 267.4	$ 128.2
Value of each common share repurchased and retired during the period (in dollars per share)		$ 803	$ 739	$ 618
Chief Executive Officer
Common Shares Issued [Abstract]
Stock Issued During Period, Shares, New Issues		14,930
Repurchased for Employee Benefit Plans [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period		8,022	10,802	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		23.90%	24.50%
Non-controlling interests		$ 244.6	$ 245.6
HG/BAM [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ (150.9)	$ (140.0)
Star & Shield Insurance Exchange [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		100.00%	100.00%
Non-controlling interests		$ 4.4	$ (16.0)
Mutual and Reciprocals [Member]
Dividends, Common Stock [Abstract]
Non-controlling interests		$ (146.5)	$ (156.0)
HG Global
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		3.10%	3.10%
Non-controlling interests		$ 16.6	$ 17.1
MediaAlpha
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		40.00%	40.00%
Non-controlling interests		$ 11.7	$ 14.4
Tranzact
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners			36.80%
Non-controlling interests			$ 79.4
WOBI [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		5.00%	3.90%
Non-controlling interests		$ 0.7	$ 0.6
Buzzmove [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		29.10%	0.00%
Non-controlling interests		$ 2.9	$ 0.0
A W G Dewar Inc [Member]
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		18.80%	19.00%
Non-controlling interests		$ 3.9	$ 3.7
SIG Preference Shares
Dividends, Common Stock [Abstract]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		0.00%	100.00%
Non-controlling interests		$ 0.0	$ 250.0
BAM Management
Dividends, Common Stock [Abstract]
Capital contributions					$ 14.5
OneBeacon
Dividends, Common Stock [Abstract]
Non-controlling interests		$ 244.6	$ 249.3

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