The portion of reinsurance receivables representing the aggregate amount of estimated amounts due as of the balance sheet date from another insurer; also called reinsurance recoverables or unpaid losses recoverable, that are expected based upon statistical projections of incurred claims and claims settlement expenses which have not yet been reported to the insurer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 6

-Article 7

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 310

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 310

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 113

-Paragraph 14, 15

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Third Party Reinsurance (One Beacon) (Details) (USD $) In Millions, unless otherwise specified							6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2013 OneBeacon Rating	Jun. 30, 2013 OneBeacon Property Catastrophe Reinsurance Program [Member]	Jun. 30, 2013 Minimum OneBeacon Property Catastrophe Reinsurance Program [Member]	Jun. 30, 2013 Maximum OneBeacon Property Catastrophe Reinsurance Program [Member]	Jun. 30, 2013 Swiss Re Group [Member] OneBeacon	Jun. 30, 2013 Olympus [Member] OneBeacon	Jun. 30, 2013 Swiss Re Group OneBeacon	Jun. 30, 2013 Michigan Catastorphic Claims Association [Member] OneBeacon	Jun. 30, 2013 Berkshire Hathaway [Member] OneBeacon
Reinsurance [Line Items]
Reinsurance recoverables											$ 6.7	$ 5.4	$ 5.0	$ 4.9	$ 10.2
Reinsurance recoverable on paid losses	16.1		17.9				2.8
Reinsurance recoverable on unpaid losses	388.6	410.8	429.1	2,369.4	2,465.4	2,507.3	81.8
Reinsurance Recoverables on Unpaid Losses							90.00%
Number of ratings in A.M. Best ratings							16
Property Catastrophe Reinsurance Program First Layer								20.0
Property Catastrophe Reinsurance Program, Second Layer								130.0
Property Catastrophe Reinsurance Program Second Layer First Retention Percentage								50.00%
Property Catastrophe Reinsurance Program First Retention Slab for Second Layer									20.0	30.0
Property Catastrophe Reinsurance Program Second Layer, Second Retention Percentage								10.00%
Property Catastrophe Reinsurance Program Second Retention Slab for Second Layer									30.0	70.0
Property Catastrophe Reinsurance Program Second Layer Third Retention Percentage								5.00%
Property Catastrophe Reinsurance Program Third Retention Slab For Second Layer									70.0	150.0
Property Catastrophe Reinsurance Retention								$ 150.0
Percentage of total paid and unpaid reinsurance recoverables											6.00%	5.00%	4.00%	4.00%	9.00%