Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) multiplier shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares
Unamortized grant date fair value
Beginning of period	$ 140.7
End of period (3)	$ 140.0	$ 140.7
Performance Shares | Range of Performance Cycle from 2011 to 2013 [Member]
Unamortized grant date fair value
Vested (in shares) | shares			3,570
Performance Shares | Range Of Performance Cycle From 2013 to 2015 [Member]
Unamortized grant date fair value
Vested (in shares) | shares	5,000
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] | Performance Cycle From 2012 to 2014
Unamortized grant date fair value
Performance goal percentage for minimum payout		91.00%
Payout for maximum growth target percentage		145.50%
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 2013 to 2015 [Member]
Unamortized grant date fair value
Performance goal percentage for minimum payout	140.00%
Payout for maximum growth target percentage	142.00%
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	93,654	111,257	108,605
Shares paid or expired(1) | shares	(36,294)	(42,959)	(33,730)
New grants | shares	22,615	29,195	39,590
Assumed forfeitures and cancellations(2) | shares	378	(3,839)	(3,208)
End of period (3) | shares	80,353	93,654	111,257
Unamortized grant date fair value
Beginning of period	$ 57.7	$ 44.4	$ 54.9
Shares paid or expired(1)	(41.0)	(30.8)	(24.5)
Assumed forfeitures and cancellations(2)	0.5	(0.3)	(0.1)
Expense recognized	25.2	44.4	14.1
End of period (3)	42.4	$ 57.7	$ 44.4
Unamortized grant date fair value	$ 23.1