XML 46 R26.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Third Party Reinsurance (Tables) | 6 Months Ended
--- | ---
Jun. 30, 2013
Reinsurance Disclosures [Abstract] |
Schedule of One Beacon Top Reinsurers [Table Text Block] | The following table provides a listing of OneBeacon's top reinsurers, excluding industry pools and associations, based upon recoverable amounts, the percentage of total recoverables and the reinsurers’ A.M. Best Rating. The reinsurance balances associated with the runoff business are included in discontinued operations (see Note 15).
Top Reinsurers (Millions) |  | Balance at June 30, 2013 |  | % of Total |  | A.M. Best Rating(1) |
Hannover Ruckversich |  | $10.2 |  | 9% |  | A+ |
Munich Reinsurance America |  | 6.7 |  | 6% |  | A+ |
Platinum Underwriters |  | 5.4 |  | 5% |  | A |
Swiss Reinsurance America Corp |  | 5.0 |  | 4% |  | A+ |
Hartford Steam Boiler |  | 4.9 |  | 4% |  | A++ |
(1) A.M. Best ratings as detailed above are: “A++” (Superior, which is the highest of sixteen financial strength ratings), “A+” (Superior, which is the second highest of sixteen financial strength ratings) and “A” (Excellent, which is the third highest of sixteen financial strength ratings).
Listing of Sirius's top reinsurers | The following table provides a listing of Sirius Group’s top reinsurers based upon recoverable amounts, the percentage of total recoverables and the reinsurers’ A.M. Best Rating.
Top Reinsurers (Millions) |  | Balance at June 30, 2013 |  | % of Total |  | A.M. Best Rating(1) |  | % Collateralized
Berkshire Hathaway |  | $56.3 |  | 18% |  | A++ |  | — | %
Swiss Re Group |  | 34.0 |  | 11% |  | A+ |  | — | %
Olympus Reinsurance Company(2) |  | 25.0 |  | 8% |  | NR-5 |  | 100 | %
Lloyds of London(3) |  | 16.1 |  | 5% |  | A |  | 16 | %
International Medical Insurance Co. Ltd. |  | 12.2 |  | 4% |  | NR-5 |  | 31 | %
(1) A.M. Best ratings as detailed above are: “A++” (Superior, which is the highest of sixteen financial strength ratings), “A+” (Superior, which is the second highest of sixteen financial strength ratings), “A” (Excellent, which is the third highest of sixteen financial strength ratings) and “NR-5” (Not formally followed). (2) Non-U.S. insurance entity. The balance is fully collateralized through trust agreements or funds held. (3) Represents the total of reinsurance recoverables due to Sirius Group from all Lloyds Syndicates.

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

X
- Definition Tabular disclosure of the top five reinsurers that have assumed business from OneBeacon (excluding industry pools and associations) based on the amount recoverable from each reinsurer. The disclosure for each reinsurer also includes the amount recoverable as a percentage of OneBeacon's consolidated reinsurance recoverables and the A.M Best Company, Inc. ("A.M. Best") rating. + References No definition available. + Details
Name: | wtm_ScheduleOfOneBeaconTopReinsurersTableTextBlock
Namespace Prefix: | wtm_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the top five reinsurers that have assumed business from Sirius based on the amount recoverable from each reinsurer. The disclosure for each reinsurer also includes the amount recoverable as a percentage of Sirius's total reinsurance recoverables, the A.M Best Company, Inc. ("A.M. Best") rating and the percentage of the amount recoverable that is collateralized. + References No definition available. + Details
Name: | wtm_ScheduleOfSiriusTopReinsurersTableTextBlock
Namespace Prefix: | wtm_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration