Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) multiplier shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($) shares
Unamortized grant date fair value
Beginning of period	$ 108.1
End of period (4)	$ 141.7	$ 108.1
Performance Shares | Range of Performance Cycle from 2011 to 2013 [Member]
Unamortized grant date fair value
Vested (in shares) | shares		3,570
Performance Shares | White Mountains
Share-based compensation arrangement by share-based payment award
Multiplier for determining payout, low end of range | multiplier	0
Multiplier for determining payout, high end of range | multiplier	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) | shares	(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%
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 | shares	113,198	108,605	108,065
Shares paid or expired(1) | shares	(42,958)	(33,730)	(43,100)
New grants | shares	29,828	41,580	43,660
Assumed forfeitures and cancellations(2)(3) | shares	(3,857)	(3,257)	(20)
End of period (4) | shares	96,211	113,198	108,605
Unamortized grant date fair value
Beginning of period	$ 44.4	$ 54.9	$ 26.5
Shares paid or expired(1)	(30.8)	(24.5)	(10.1)
Assumed forfeitures and cancellations(2)(3)	(0.3)	(0.1)	(0.6)
Expense recognized	44.4	14.1	39.1
End of period (4)	57.7	$ 44.4	$ 54.9
Unamortized grant date fair value	$ 22.5