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Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (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
OneBeacon | Performance Shares | Long Term Incentive Plan
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
Unamortized grant date fair value	$ 2.1
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	12.00%	13.00%	14.00%
Range Of Performance Cycle From 2016-2018 [Member] | OneBeacon | Performance Shares | Long Term Incentive Plan
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
Range Of Performance Cycle From 2015 to 2017 [Member] | OneBeacon | Performance Shares | Long Term Incentive Plan
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
Range Of Performance Cycle From 2014-2016 [Member] | OneBeacon | Performance Shares | Long Term Incentive Plan
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
Maximum | OneBeacon | Performance Shares | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	19.00%	20.00%	21.00%
Percentage of performance shares earned	200.00%	200.00%	200.00%
Minimum | OneBeacon | Performance Shares | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	5.00%	6.00%	7.00%

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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