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SCHEDULE IV REINSURANCE (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Premiums earned |  |  |
Gross amount | $ 1,433.7 us-gaap_DirectPremiumsEarned |  |
Ceded to other companies | (371.1)us-gaap_CededPremiumsEarned |  |
Assumed from other companies | 996.3 us-gaap_AssumedPremiumsEarned |  |
Net earned premiums | 2,058.9 us-gaap_PremiumsEarnedNet | 1,987.3 us-gaap_PremiumsEarnedNet | 2,063.6 us-gaap_PremiumsEarnedNet
OneBeacon |  |  |
Premiums earned |  |  |
Gross amount | 1,209.1 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,043.3 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,158.3 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Ceded to other companies | (102.9)us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | (71.4)us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | (79.1)us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Assumed from other companies | 70.9 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 148.5 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 52.8 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Net earned premiums | 1,177.1 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,120.4 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,132.0 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Percentage of amount assumed to net | 6.00%us-gaap_PremiumsPercentageAssumedToNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 13.30%us-gaap_PremiumsPercentageAssumedToNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 4.70%us-gaap_PremiumsPercentageAssumedToNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Sirius Group |  |  |
Premiums earned |  |  |
Gross amount | 200.2 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | 174.0 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | 169.9 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember
Ceded to other companies | (251.7)us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | (246.2)us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | (226.6)us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember
Assumed from other companies | 925.4 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | 938.6 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | 988.3 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember
Net earned premiums | 873.9 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | 866.4 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | 931.6 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember
Percentage of amount assumed to net | 105.90%us-gaap_PremiumsPercentageAssumedToNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | 108.30%us-gaap_PremiumsPercentageAssumedToNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember | 106.10%us-gaap_PremiumsPercentageAssumedToNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupSegmentMember
HG/BAM |  |  |
Premiums earned |  |  |
Gross amount | 1.8 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Ceded to other companies | 0 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Assumed from other companies | 0 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Net earned premiums | 1.8 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Percentage of amount assumed to net | 0.00%us-gaap_PremiumsPercentageAssumedToNet / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Other Segments |  |  |
Premiums earned |  |  |
Gross amount | 22.6 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Ceded to other companies | (16.5)us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Assumed from other companies | 0 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Net earned premiums | $ 6.1 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | $ 0 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | $ 0 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Percentage of amount assumed to net | 0.00%us-gaap_PremiumsPercentageAssumedToNet / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |

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- Definition Amount of earned premiums assumed from other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 + Details
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- Definition Amount of earned premiums ceded to other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486662&loc=d3e27175-158546 + Details
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- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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