Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 140.0	$ 140.7
Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	9.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2013 to 2015 | Performance Shares
Share-based compensation arrangement by share-based payment award
Vested (in shares)	(5,000)
Performance Cycle From 2013 to 2015 | W T M Incentive and Phantom Plan [Member] | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	140.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	142.00%
Performance Cycle From 2012 to 2014 | W T M Incentive and Phantom Plan [Member] | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout		91.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage		145.50%
OneBeacon | Long Term Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Shares outstanding (in shares)	452,519	449,435	517,470	493,421
Accrued incentive compensation	$ 1.6	$ 1.4	$ 3.4	$ 4.0
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	12.00%	13.00%	14.00%
OneBeacon | Range Of Performance Cycle From 2016-2018 [Member] | Long Term Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	163,150
Incentive compensation payable before forfeitures	$ 0.9
OneBeacon | Range Of Performance Cycle From 2015 to 2017 [Member] | Long Term Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	146,659
Incentive compensation payable before forfeitures	$ 0.7
OneBeacon | Range Of Performance Cycle From 2014-2016 [Member] | Long Term Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	142,710
Incentive compensation payable before forfeitures	$ 0.0
White Mountains | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	81,577
Incentive compensation payable before forfeitures	$ 43.1
forfeitures	(1,224)
Assumed forfeitures in period amount	(0.7)
Shares outstanding (in shares)	80,353	93,654	111,257	108,605
Accrued incentive compensation	$ 42.4	$ 57.7	$ 44.4	$ 54.9
White Mountains | Range Of Performance Cycle From 2016-2018 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	19,615
Incentive compensation payable before forfeitures	$ 5.4
White Mountains | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	31,795
Incentive compensation payable before forfeitures	$ 19.1
White Mountains | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	30,167
Incentive compensation payable before forfeitures	$ 18.6
Sirius Group | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Shares outstanding (in shares)	7,315	10,826	12,607
Minimum | Range Of Performance Cycle From 2016-2018 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	0.00%
Minimum | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Minimum | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	3.00%
Minimum | OneBeacon | Long Term Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	5.00%	6.00%	7.00%
Maximum | Range Of Performance Cycle From 2016-2018 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Maximum | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%
Maximum | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	15.00%
Maximum | OneBeacon | Long Term Incentive Plan | Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	19.00%	20.00%	21.00%