XML 182 R69.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Third Party Reinsurance (Written and Earned Premiums) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Written premiums: |  |  |
Direct | $ 1,505.0 us-gaap_DirectPremiumsWritten |  |
Assumed | 993.8 us-gaap_AssumedPremiumsWritten |  |
Gross written premiums | 2,498.8 us-gaap_PremiumsWrittenGross |  |
Ceded | 377.3 us-gaap_CededPremiumsWritten |  |
Net written premiums | 2,121.5 us-gaap_PremiumsWrittenNet |  |
Earned premiums: |  |  |
Gross amount | 1,433.7 us-gaap_DirectPremiumsEarned |  |
Assumed from other companies | 996.3 us-gaap_AssumedPremiumsEarned |  |
Gross earned premiums | 2,430.0 wtm_PremiumsEarnedGross |  |
Ceded | 371.1 us-gaap_CededPremiumsEarned |  |
Net earned premiums | 2,058.9 us-gaap_PremiumsEarnedNet | 1,987.3 us-gaap_PremiumsEarnedNet | 2,063.6 us-gaap_PremiumsEarnedNet
Losses and LAE: |  |  |
Direct | 972.6 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross |  |
Policyholder Benefits and Claims Incurred, Assumed | 441.8 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed |  |
Gross losses and LAE | 1,414.4 wtm_GrossPolicyholderBenfitsAndClaimsIncurred |  |
Ceded | (245.1)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded |  |
Total incurred losses and LAE | 1,169.3 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet | 1,040.5 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet | 1,193.9 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet
OneBeacon |  |  |
Written premiums: |  |  |
Direct | 1,257.5 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,103.1 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,204.0 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Assumed | 65.9 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 59.8 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 55.2 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Gross written premiums | 1,323.4 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,162.9 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,259.2 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Ceded | 106.5 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 74.3 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 80.0 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Net written premiums | 1,216.9 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,088.6 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,179.2 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Earned premiums: |  |  |
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
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
Gross earned premiums | 1,280.0 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,191.8 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,211.1 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Ceded | 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
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
Losses and LAE: |  |  |
Direct | 830.7 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 584.9 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 687.5 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Policyholder Benefits and Claims Incurred, Assumed | 63.7 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 76.3 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 29.6 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Gross losses and LAE | 894.4 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 661.2 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 717.1 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Ceded | (79.3)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | (39.1)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | (67.1)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Total incurred losses and LAE | 815.1 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 622.1 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 650.0 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Sirius Group |  |  |
Written premiums: |  |  |
Direct | 208.7 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 177.3 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 186.1 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Assumed | 927.9 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 943.1 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 992.7 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Gross written premiums | 1,136.6 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 1,120.4 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 1,178.8 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Ceded | 254.1 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 243.8 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 231.1 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Net written premiums | 882.5 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 876.6 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 947.7 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Earned premiums: |  |  |
Gross amount | 200.2 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 174.0 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 169.9 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Assumed from other companies | 925.4 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 938.6 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 988.3 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Gross earned premiums | 1,125.6 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 1,112.6 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 1,158.2 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Ceded | 251.7 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 246.2 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 226.6 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Net earned premiums | 873.9 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 866.4 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 931.6 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Losses and LAE: |  |  |
Direct | 117.8 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 98.1 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 96.9 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Policyholder Benefits and Claims Incurred, Assumed | 378.1 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 455.5 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 523.9 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Gross losses and LAE | 495.9 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 553.6 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 620.8 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Ceded | (150.6)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (135.2)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (76.9)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Total incurred losses and LAE | 345.3 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 418.4 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 543.9 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
HG/BAM |  |  |
Written premiums: |  |  |
Direct | 16.2 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Assumed | 0 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Gross written premiums | 16.2 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Ceded | 0 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Net written premiums | 16.2 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Earned premiums: |  |  |
Gross amount | 1.8 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Assumed from other companies | 0 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Gross earned premiums | 1.8 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Ceded | 0 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Net earned premiums | 1.8 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Losses and LAE: |  |  |
Direct | 0 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Policyholder Benefits and Claims Incurred, Assumed | 0 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Gross losses and LAE | 0 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Ceded | 0 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
Total incurred losses and LAE | 0 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_HGBAMMember |  |
HG/BAM [Member] |  |  |
Written premiums: |  |  |
Ceded | 12.3 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_BuildAmericaMutualMember |  |
Earned premiums: |  |  |
Ceded | 1.4 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_BuildAmericaMutualMember |  |
Star & Shield Insurance Exchange [Member] |  |  |
Written premiums: |  |  |
Ceded | 16.0 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_StarShieldInsuranceExchangeMember |  |
Earned premiums: |  |  |
Ceded | 15.7 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_StarShieldInsuranceExchangeMember |  |
Other Segments |  |  |
Written premiums: |  |  |
Direct | 22.6 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Assumed | 0 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Gross written premiums | 22.6 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Ceded | 16.7 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Net written premiums | 5.9 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Earned premiums: |  |  |
Gross amount | 22.6 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Assumed from other companies | 0 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Gross earned premiums | 22.6 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Ceded | 16.5 us-gaap_CededPremiumsEarned / 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
Losses and LAE: |  |  |
Direct | 24.1 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Policyholder Benefits and Claims Incurred, Assumed | 0 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Gross losses and LAE | 24.1 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Ceded | (15.2)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Total incurred losses and LAE | 8.9 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 0 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 0 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
OneBeacon and Sirius Group |  |  |
Written premiums: |  |  |
Direct |  | 1,280.4 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 1,390.1 us-gaap_DirectPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Assumed |  | 1,002.9 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 1,047.9 us-gaap_AssumedPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Gross written premiums |  | 2,283.3 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 2,438.0 us-gaap_PremiumsWrittenGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Ceded |  | 318.1 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 311.1 us-gaap_CededPremiumsWritten / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Net written premiums |  | 1,965.2 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 2,126.9 us-gaap_PremiumsWrittenNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Earned premiums: |  |  |
Gross amount |  | 1,217.3 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 1,328.2 us-gaap_DirectPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Assumed from other companies |  | 1,087.1 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 1,041.1 us-gaap_AssumedPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Gross earned premiums |  | 2,304.4 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 2,369.3 wtm_PremiumsEarnedGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Ceded |  | 317.6 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 305.7 us-gaap_CededPremiumsEarned / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Net earned premiums |  | 1,986.8 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 2,063.6 us-gaap_PremiumsEarnedNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Losses and LAE: |  |  |
Direct |  | 683.0 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 784.4 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Policyholder Benefits and Claims Incurred, Assumed |  | 531.8 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 553.5 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Gross losses and LAE |  | 1,214.8 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | 1,337.9 wtm_GrossPolicyholderBenfitsAndClaimsIncurred / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Ceded |  | (174.3)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | (144.0)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember
Total incurred losses and LAE |  | $ 1,040.5 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember | $ 1,193.9 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = wtm_OneBeaconandSiriusGroupMember

X
- 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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of written premiums assumed from other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(2),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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