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Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015
Investment securities |  |
Investments held in trusts | $ 165,900,000 | $ 143,600,000
Assets held by insurance regulators | $ 93,500,000 | $ 88,000,000
Percentage of investments recorded at fair value | 94.00% | 91.00%
Surety Product Line [Member] | OneBeacon |  |
Investment securities |  |
Assets Held in Collateral | $ 153,000,000 | $ 137,700,000
Private equity funds |  |
Investment securities |  |
Other than Temporary Impairment Losses, Investments | 5,000,000 | $ 2,400,000.0
Fixed maturity investments | White Mountains Life Re |  |
Investment securities |  |
Derivative Collateral Right to Reclaim | $ 0 |

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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 The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560 + Details
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- Definition Cash and investments held in collateral. + References No definition available. + Details
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