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Transactions with Related Persons (Details) (USD $) | 12 Months Ended |  | 12 Months Ended | 1 Months Ended | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended
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Dec. 31, 2011 Prospector | Dec. 31, 2010 Prospector | Dec. 31, 2009 Prospector | Dec. 31, 2011 Prospector Minimum | Dec. 31, 2011 Prospector Maximum | Dec. 31, 2011 Prospector Performance cycle 2012-2014 | Dec. 31, 2010 Prospector Performance cycle 2011-2013 | Dec. 31, 2009 Prospector Performance cycle 2010-2012 | Feb. 29, 2012 Prospector Performance cycle 2009-2011 | Dec. 31, 2009 Prospector Performance cycle 2009-2011 | Dec. 31, 2011 Prospector OneBeacon | Dec. 31, 2011 Symetra | Dec. 31, 2010 Symetra | Dec. 31, 2009 Symetra | Jan. 07, 2011 Franklin Mutual Advisers | Dec. 31, 2009 Dowling & Partners Connecticut Fund III, LP (Fund III) Mr. John Gillespie | Dec. 31, 2011 Dowling & Partners Connecticut Fund III, LP (Fund III) Mr. John Gillespie entity | Dec. 31, 2011 WM Advisors Prospector | Dec. 31, 2006 OneBeacon Professional Insurance | Dec. 31, 2006 White Mountains Specialty Underwriting, Inc.
Transactions with Related Persons |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Advisors fees charged on first $200 million of assets under management (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.00% |  |
First threshold limit of assets under management |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000,000 |  |
Advisors fees charged on second $200 million of assets under management (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.50% |  |
Second threshold limit of assets under management |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200,000,000 |  |
Advisors fees charged on over $400 million of assets under management (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.25% |  |
Third threshold limit of assets under management |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000,000 |  |
Assets under management |  |  |  |  |  |  |  |  |  |  | 558,200,000 | 26,300,000,000 | 23,600,000,000 |  |  |  |  | 408,200,000 |  |
ERISA Assets |  |  |  |  |  |  |  |  |  |  | 216,600,000 |  |  |  |  |  |  |  |  |
Asset management total fees earned | 6,000,000 | 6,600,000 | 6,500,000 |  |  |  |  |  |  |  |  | 15,200,000 | 13,800,000 | 13,700,000 |  |  |  |  |  |
Percentage of target number granted actually received by prospector |  |  |  |  |  |  |  |  | 151.00% |  |  |  |  |  |  |  |  |  |  |
Number of performance shares granted |  |  |  |  |  | 7,750 | 8,500 | 9,200 |  | 10,800 |  |  |  |  |  |  |  |  |  |
Target number of performance shares granted (as a percent) |  |  |  | 0.00% | 200.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment made with respect to the performance shares granted to the related party |  |  |  |  |  |  |  |  | 7,700,000 |  |  |  |  |  |  |  |  |  |  |
Performance shares granted at the beginning of each performance cycle | 4,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fees in excess of $500,000 revenue chargeable | 6.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of revenue over which 6% of fees chargeable | 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue sharing agreement fee earned | 200,000 | 700,000 | 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Invested in limited partnership investment interests | 109,100,000 |  |  |  |  |  |  |  |  |  | 42,900,000 |  |  |  |  |  |  |  |  |
Management fees incurred |  |  |  |  |  |  |  |  |  |  | 1,100,000 |  |  |  |  |  |  |  |  |
Incentive fees incurred |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Number of shares repurchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 140,000 |  |  |  |  |
Purchase price per share of shares repurchased (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 342.50 |  |  |  |  |
Excess of market price over purchase price per share of shares repurchased (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.50 |  |  |  |  |
Interest in Dowling & Partners Connecticut Fund III, LP (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33.00% |  |  |
Number of indirect subsidiaries borrowing from Fund III |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |
Borrowings from Fund III |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,000,000 | 7,000,000
Tax credits generated by related party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000 |  |  |  |

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- Definition Revenue recognized in the period for (1) performance (incentive) fees based on the investment results achieved for management of certain institutional accounts and hedge funds, (2) schedule-based fees earned for management of mutual funds and closed-end funds, based either on average daily net assets or on a combination of the average daily net assets and gross income, and (3) other investment management and advisory fees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-BRD -Chapter 4 -Paragraph 9, 51, 59 -IssueDate 2006-05-01 -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. Reference 2: 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(a),(d)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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- Definition The amount deducted from an entities' taxes, reflecting a percentage of an amount invested in certain assets spread over the useful life of those assets; includes deferred investment tax credits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition The cash outflow for the payment of a long-term borrowing made from a related party where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth. Alternate caption: Payments for Advances from Affiliates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3291-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 18 -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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 20 -Subparagraph b -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. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(c) -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Represents the amount of revenue over which the specified percentage of fees is chargeable pursuant to revenue sharing agreement with the related party. + References No definition available. + Details
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- Definition The amount of assets an investment adviser manages on behalf of investors related to its defined benefit and defined contribution plans. + References No definition available. + Details
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- Definition Represents the first specified threshold limit of assets under management on which the specified percentage of investment advisory fees is levied. + References No definition available. + Details
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- Definition Represents the percentage of investment advisory fees levied by the investment adviser on first specified threshold limit of assets under management. + References No definition available. + Details
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- Definition Represents the percentage of investment advisory fees levied by the investment adviser on second specified threshold limit of assets under management. + References No definition available. + Details
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- Definition Represents the percentage of investment advisory fees levied by the investment adviser on over third specified threshold limit of assets under the management. + References No definition available. + Details
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- Definition The number of units or percentage investment held in the LLC or LP. + References No definition available. + Details
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- Definition Represents the percentage of fees in excess of the specified amount of revenue is chargeable pursuant to revenue sharing agreement with the related party. + References No definition available. + Details
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- Definition Represents the excess of market price per share over the price at which the the entity has agreed to repurchase the shares from the transactions with related party. + References No definition available. + Details
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- Definition Incentive fees recognized during the period resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period. + References No definition available. + Details
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- Definition Management fees recognized during the period resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period. + References No definition available. + Details
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- Definition Represents the number of shares repurchased by the entity from transactions with related party during the period. + References No definition available. + Details
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- Definition Represents the purchase price per share of shares agreed to be repurchased by the entity from transactions with related party during the period. + References No definition available. + Details
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- Definition Represents the second specified threshold limit of assets under management on which specified percentage of investment advisory fees levied. + References No definition available. + Details
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- Definition Represents the value of performance shares granted at the beginning of each performance cycle to the related party by the entity. + References No definition available. + Details
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- Definition Represents the payment made with respect to performance shares granted to the related party. + References No definition available. + Details
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- Definition Represents the percentage of performance shares that are actually awarded as a percentage of the target grant. + References No definition available. + Details
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- Definition Represents the percentage of performance shares that may be awarded as a percentage of the target grant. + References No definition available. + Details
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- Definition Represents third specified threshold limit of assets under management over which specified percentage of insurance advisory fees levied. + References No definition available. + Details
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