Common Shareholders' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Aug. 26, 2010	Nov. 17, 2006
Common Shares Repurchased and Retired [Abstract]
Authorized repurchase of common shares	2,600,000				1,000,000
Additional authorized repurchase of common shares	1,000,000			600,000
Shares remaining under share repurchase authorization	685,496,000
Common shares repurchased and retired during the period	1,329,640	646,502	687,871
Number of Modified Dutch Auction	2
Value of common shares repurchased and retired during the period	$ 669.1	$ 253.0	$ 225.5
Value of each common share repurchased and retired during the period (in dollars per share)	$ 503	$ 390	$ 328
Common Shares Issued [Abstract]
Common shares issued, total (in shares)	44,054	29,432	23,050
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period			2,400
Shares issued in satisfaction of options exercised (in shares)	32,160	27,250	19,750
Shares issued to directors (in shares)	1,617	2,182	900
Dividends, Common Stock [Abstract]
Cash dividends declared and paid	6.6	8.0	8.8
Dividends delared and paid (in dollars per share)		$ 1	$ 1
General Board Authorization
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	502,801	313,967	677,125
Value of common shares repurchased and retired during the period	256.0	114.0	222.0
Value of each common share repurchased and retired during the period (in dollars per share)	$ 508	$ 364	$ 264
Tender Offer
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	816,829	332,346
Value of common shares repurchased and retired during the period	408.6	138.8
Value of each common share repurchased and retired during the period (in dollars per share)	$ 500	$ 418
Shares Repurchased For Employee Benefit Plan [Member]
Common Shares Repurchased and Retired [Abstract]
Common shares repurchased and retired during the period	10,010	189	10,746
Non-controlling Interests
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	0	0	0
Total Equity includes Noncontrolling Interest [Member]
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	669.1	253.0	225.6
White Mountains | Common shares and paid-in surplus
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	221.3	107.2	112.4
White Mountains | Retained earnings
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	447.8	145.8	113.2
White Mountains | AOCI, after-tax
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	0	0	0
White Mountains | Common shareholders’ equity
Common Shares Repurchased and Retired [Abstract]
Value of common shares repurchased and retired during the period	$ 669.1	$ 253.0	$ 225.6