Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) (USD $)	12 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2012
Unamortized grant date fair value
Ending December 31,	184,600,000	218,300,000	184,600,000
Performance Shares | Range of Performance Cycle from 2011 to 2013 [Member]
Unamortized grant date fair value
Vested (in shares)	3,570
Performance Shares | Range of Performance Cycle from 2009 to 2011 [Member]
Unamortized grant date fair value
Vested (in shares)		9,577
Performance Shares | White Mountains
Share-based compensation arrangement by share-based payment award
Multiplier for determining payout, low end of range	0
Multiplier for determining payout, high end of range	2
Performance cycle period	3 years
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range Of Performance Cycle From 2014-2016 [Member]
Target Performance Share activity
Assumed forfeitures and cancellations(2)(3)			3,040
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range of Performance Cycle from 2011 to 2013 [Member]
Unamortized grant date fair value
Performance goal percentage for minimum payout	88.00%
Payout for maximum growth target percentage	131.50%
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range of Performance Cycle from 2010 to 2012 [Member]
Unamortized grant date fair value
Performance goal percentage for minimum payout		33.00%
Payout for maximum growth target percentage		98.00%
Partial Payout for 2010 - 2012 cycle				9.9
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range of Performance Cycle from 2009 to 2011 [Member]
Unamortized grant date fair value
Performance goal percentage for minimum payout				147.00%
Payout for maximum growth target percentage				155.00%
Performance Shares | W T M Incentive Plan | Range Of Performance Cycle From 2014-2016 [Member]
Unamortized grant date fair value
Payout for maximum growth target percentage	200.00%
Performance Shares | W T M Incentive Plan | White Mountains
Target Performance Share activity
Beginning of period	119,220	119,357		150,064
Shares paid or expired(1)	(37,130)	(47,310)		(68,357)
New grants	45,660	47,170		38,432
Assumed forfeitures and cancellations(2)(3)	4,201	(3)		782
Ending December 31,	123,549	119,220	123,549	119,357
Unamortized grant date fair value
Beginning of period	60,200,000	29,400,000		66,100,000
Shares paid or expired(1)	(26,700,000)	(11,000,000)		(58,300,000)
Assumed forfeitures and cancellations(2)(3)	(700,000)	(600,000)		600,000
Expense recognized	15,300,000	42,400,000		21,000,000
Ending December 31,	48,100,000	60,200,000	48,100,000	29,400,000
Unamortized grant date fair value	16,900,000		16,900,000