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Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract] |  |
Investments held in trusts | $ 204.6 | $ 161.7
Assets held by insurance regulators | $ 6.0 | $ 6.0
Percentage of investments recorded at fair value | 94.00% | 94.00%

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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=75027294&loc=d3e22002-158495 + Details
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- References No definition available. + Details
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- Definition Investments Held In Trusts + References No definition available. + Details
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- Definition Represents the percentage of the total investment portfolio recorded at fair value based upon quoted market prices or other observable inputs. + References No definition available. + Details
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