XML 90 R70.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Third-Party Reinsurance (OneBeacon) (Details) - USD ($) $ in Millions | 3 Months Ended | 5 Months Ended | 6 Months Ended | 7 Months Ended | 12 Months Ended | 60 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2015 | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2015 | Dec. 31, 2020 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2020 | Dec. 31, 2013 | Dec. 31, 2012
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | $ 186.5 | $ 186.5 |  | $ 186.5 |  |  | $ 186.5 | $ 161.7 |  | $ 80.2 | $ 107.3
OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Catastrophe Event Ratio |  |  | 0.40% |  |  |  |  |  |  |  |
OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | 7.5 | 7.5 |  | 7.5 |  |  | 7.5 |  |  |  |
Reinsurance recoverable on unpaid losses | 186.0 | $ 186.0 |  | 186.0 |  |  | $ 186.0 |  |  |  |
Property Catastrophe Program [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  |  |  | 95.00% |  |  |  |
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation |  |  |  |  |  |  | $ 10.0 |  |  |  |
Property Catastrophe Program [Member] | Maximum | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 130.0 |  |  |  |
Property Catastrophe Program [Member] | Minimum | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  | $ 20.0 |  |  |  |
Second Casualty Excess of Loss [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  |  |  | 100.00% |  |  |  |
Property Per-Risk, Acts of Terrorism | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  |  |  | 100.00% |  |  |  |
Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | $ 100,000.0 |  |  |  |
Property Per-Risk, Acts of Terrorism | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 100.0 |  |  |  |
Property Per-Risk, Acts of Terrorism | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 3.0 |  |  |  |
Property Per Risk Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 100.0 |  |  |  |
Property Per Risk Reinsurance Program | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 3.0 | $ 5.0 |  |  |
Individual Risk Facultative [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 100.0 |  |  |  |
Specialty Property | Property Catastrophe Reinsurance [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | $ 34.0 |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  |  |  | 100.00% |  |  |  |
Specialty Property | Property Catastrophe Reinsurance [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  | $ 6.0 |  |  |  |
Inland and Ocean Marine | Marine Treaty Per Occurrence | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  | 2.5 |  |  |  |
Inland and Ocean Marine | Marine Treaty | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 60.0 |  |  |  |
Inland and Ocean Marine | Layer One | Property Per Risk Treaty | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  | 20.0 |  |  |  |
Inland and Ocean Marine | Layer One | Catastrophic Losses [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  | 2.5 |  |  |  |
Annual Aggregate Deductible to Reinsurance Retention Policy |  |  |  |  |  |  | 5.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 7.5 |  |  |  |
Health Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 10.0 |  |  |  |
Health Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 3.0 |  |  |  |
Health Insurance Product Line | HMO Provider Excess Reinsurance [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 5.0 |  |  |  |
Property, Liability and Casualty Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 11.0 |  |  |  |
Property, Liability and Casualty Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | $ 3.0 |  |  |  |
Workers compensation | Casualty Reinsurance Program | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  |  |  | 100.00% |  |  |  |
Workers compensation | Casualty Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | $ 10.0 |  |  |  |
Workers compensation | Casualty Reinsurance Program | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 2.0 |  |  |  |
Workers compensation | Dedicated Clash Treaty [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  | 10.0 |  |  |  |
Workers compensation | Dedicated Clash Treaty [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 60.0 |  |  |  |
Casualty and Surety [Member] | Dedicated Clash Treaty [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  | 10.0 |  |  |  |
Casualty and Surety [Member] | Dedicated Clash Treaty [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 40.0 |  |  |  |
Healthcare Professional Liability [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 20.0 |  |  |  |
Healthcare Professional Liability [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 10.0 |  |  |  |
Other Casualty Business [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 21.0 |  |  |  |
Other Casualty Business [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  | 11.0 |  |  |  |
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | $ 100.0 |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 100.00% |  |  |  |  |  |  |  |  |  |
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | $ 50.0 |  |  |  |  |  |  |  |  |  |
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5.0 |  |  |  |  |  |  |  |  |  |
Crop | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage |  | 100.00% |  |  |  |  |  |  |  |  |
Film Completion Bond Business [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 35.0 |  |  |  |  |  |  |
Film Completion Bond Business [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 2.0 |  |  |  |  |  |  |
Film Completion Bond Business [Member] | Facultative Treaty Layer [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 60.0 |  |  |  |  |  |  |
OneBeacon Runoff | BBB | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 20.4 | $ 20.4 |  | 20.4 |  |  | 20.4 |  |  |  |
Scenario, Forecast [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act |  |  |  |  | 80.00% | 84.00% |  |  |  |  |
Decreasing Annual Percentage of Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act |  |  |  |  |  |  |  |  | 1.00% |  |
Retained Percentage Relating To Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act |  |  |  |  | 20.00% | 16.00% |  |  |  |  |
Increased Annual Retained Percentage Relating to Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act |  |  |  |  |  | 1.00% |  |  |  |  |
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  | $ 120.0 |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  |  | 84.00% |  |  |  |  |
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  | $ 100,000.0 |  |  |  |  |
Department of Treasury [Member] | Scenario, Forecast [Member] | Property and Casualty, Commercial Insurance Product Line [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  | $ 200.0 | $ 120.0 |  |  | $ 20.0 |  |
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 193.5 | 193.5 |  | 193.5 |  |  | $ 193.5 |  |  |  |
Reinsurance recoverables, percentage of total |  |  |  |  |  |  | 100.00% |  |  |  |
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AA | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 41.7 | 41.7 |  | 41.7 |  |  | $ 41.7 |  |  |  |
Reinsurance recoverables, percentage of total |  |  |  |  |  |  | 22.00% |  |  |  |
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | A | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 126.2 | 126.2 |  | 126.2 |  |  | $ 126.2 |  |  |  |
Reinsurance recoverables, percentage of total |  |  |  |  |  |  | 65.00% |  |  |  |
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | BBB | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | $ 25.6 | $ 25.6 |  | $ 25.6 |  |  | $ 25.6 |  |  |  |
Reinsurance recoverables, percentage of total |  |  |  |  |  |  | 13.00% |  |  |  |
Expiration March 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member] | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | $ 7,500.0 |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  | 5.0 |  |  |  |  |  |  |  |
Expiration March 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member] | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | 5,000.0 |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  | 5.0 |  |  |  |  |  |  |  |
Expiration December 2015 [Member] | All Natural Perils [Member] | Industry Loss Warranty [Member] |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | 15,000.0 |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  | 15.0 |  |  |  |  |  |  |  |
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  | 30.0 |  |  |  |  |  |  |  |
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | 10,000.0 |  |  |  |  |  |  |  |
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | 5,000.0 |  |  |  |  |  |  |  |
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  | 7.5 |  |  |  |  |  |  |  |
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | 7,500.0 |  |  |  |  |  |  |  |
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | 5,000.0 |  |  |  |  |  |  |  |
Expiration June 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member] |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | 20,000.0 |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  | $ 5.0 |  |  |  |  |  |  |  |

X
- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=51801978&loc=d3e6351-108592 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13531-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=51801978&loc=d3e6404-108592 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13537-108611 + Details
Name: | us-gaap_ConcentrationRiskPercentage1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | us-gaap_EffectsOfReinsuranceLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Estimated amount after valuation allowance of reinsurance recoverables, due from reinsurers for incurred claims not yet reported, and claims settlement expenses. + References Reference 1: 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 2: 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 + Details
Name: | us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount after valuation allowance currently billed or billable under reinsurance contracts. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 4: 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 + Details
Name: | us-gaap_ReinsuranceReceivablesPaidLossesRecoverable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_ReinsuranceRecoverables
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Stated amount that is the upper retention limit per entity policy. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRetentionPolicyAmountRetained
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of risk reinsured in excess of stated retention per company policy. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage of risk reinsured on insurance products per entity policy. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Annual Aggregate Deductible to Reinsurance Retention Policy + References No definition available. + Details
Name: | wtm_AnnualAggregateDeductibletoReinsuranceRetentionPolicy
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Catastrophe Event Ratio + References No definition available. + Details
Name: | wtm_CatastropheEventRatio
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Decreasing Annual Percentage of Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act + References No definition available. + Details
Name: | wtm_DecreasingAnnualPercentageofCertifiedLossesRequiredtobeSharedbyFederalGovernmentSubjecttoTerrorismRiskInsuranceAct
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Increased Annual Retained Percentage Relating to Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act + References No definition available. + Details
Name: | wtm_IncreasedAnnualRetainedPercentageRelatingtoCertifiedLossesRequiredtobeSharedbyFederalGovernmentSubjecttoTerrorismRiskInsuranceAct
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of certified losses required to be shared by federal government once a company meets certain conditions. + References No definition available. + Details
Name: | wtm_PercentageOfCertifiedLossesRequiredToBeSharedByFederalGovernmentSubjectToTerrorismRiskInsuranceAct
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation + References No definition available. + Details
Name: | wtm_ReinsuranceRetentionPolicyExcessRetentionAmountReinsuredNetOfParticipation
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage + References No definition available. + Details
Name: | wtm_ReinsuranceRetentionPolicyLossRatioReinsuredriskPercentage
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Retained Percentage Relating To Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act + References No definition available. + Details
Name: | wtm_RetainedPercentageRelatingToCertifiedLossesRequiredtobeSharedbyFederalGovernmentSubjecttoTerrorismRiskInsuranceAct
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_OnebeaconMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_OnebeaconMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_PropertyCatastropheProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_SecondCasualtyExcessofLossMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_PropertyPerRiskActsofTerrorismMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_PropertyPerRiskReinsuranceProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_IndividualRiskFacultativeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_SpecialtyPropertyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_PropertyCatastropheReinsuranceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_InlandandOceanMarineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_MarineTreatyPerOccurrenceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_MarineTreatyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_ReinsuranceTypeandTierIdentifierAxis=wtm_LayerOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_PropertyPerRiskTreatyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_CatastrophicLossesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=us-gaap_HealthInsuranceProductLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_CasualtyReinsuranceProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_HMOProviderExcessReinsuranceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_WorkersCompensationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_DedicatedClashTreatyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_CasualtyandSuretyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_HealthcareProfessionalLiabilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_OtherCasualtyBusinessMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=us-gaap_SuretySegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_SuretyReinsuranceProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_CropinsuranceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_FilmCompletionBondBusinessMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_FacultativeTreatyLayerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=wtm_OnebeaconRunoffMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditRatingStandardPoorsAxis=us-gaap_StandardPoorsBBBRatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=wtm_DepartmentofTreasuryMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ConcentrationRiskByTypeAxis=us-gaap_ReinsurerConcentrationRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_ReinsuranceRecoverableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditRatingStandardPoorsAxis=us-gaap_StandardPoorsAARatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditRatingStandardPoorsAxis=us-gaap_StandardPoorsARatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_ExpirationPeriodAxis=wtm_ExpirationMarch2015Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_ReinsuranceContractByNatureAxis=wtm_WindandfloodMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_IndustryLossWarrantyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_ExpirationPeriodAxis=wtm_ExpirationDecember2015Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_ReinsuranceContractByNatureAxis=wtm_AllNaturalPerilsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_ReinsuranceContractByNatureAxis=wtm_WindandearthquakeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_ExpirationPeriodAxis=wtm_ExpirationMarch2016Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_ExpirationPeriodAxis=wtm_ExpirationJune2015Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |