Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
One Beacon Employee Stock Ownership Plan
Share-based compensation arrangement by share-based payment award
Defined Contribution Plan, Cost Recognized	$ 5.5	$ 5.7	$ 4.4
Qualified Plan [Member]
Share-based compensation arrangement by share-based payment award
Eligible contribution percentage	3.00%	3.00%	2.00%
Percentage of common shares outstanding	3.00%	3.00%

The amount of the cost recognized during the period for defined contribution plans.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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Balance Type:	debit
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Represents the variable contribution amounts earned by eligible participants of the KSOP as a percentage of salary.

+ References

No definition available.

+ Details

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- Definition

Represents the percentage of total common shares outstanding owned by the plans.

+ References

No definition available.

+ Details

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- Details

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- Details

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