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Investment Securities (Investments Held on Deposit or as Collateral) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Investment securities |  |
Investments held in trusts | $ 170.4 | $ 169.9
Assets held by insurance regulators | 290.7 | 319.3
Surety Product Line [Member] | OneBeacon |  |
Investment securities |  |
Assets Held in Collateral | 63.3 | 5.9
Fixed maturity investments | White Mountains Life Re |  |
Investment securities |  |
Derivative Collateral Right to Reclaim | 23.2 | 138.7
Short-term investments | White Mountains Life Re |  |
Investment securities |  |
Derivative Collateral Right to Reclaim | $ 2.0 | $ 5.1

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- Definition Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 320 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28366721&loc=d3e22002-158495 + Details
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- Definition Cash and investments held in collateral. + References No definition available. + Details
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- Definition The amount of collateral held against amounts due to counterparties for over the counter derivative instruments against net derivative instrument positions. + References No definition available. + Details
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- Definition Investments Held In Trusts + References No definition available. + Details
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