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Reinsurance - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Prepaid Reinsurance Premiums [Abstract] |  |  |
Policyholder Benefits and Claims Incurred, Ceded | $ 1,094.2 | $ 764.4 | $ 457.3

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- Definition Prepaid Reinsurance Premiums [Abstract] + References No definition available. + Details
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Period Type: | duration

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- Definition Amount of reduction of provision for policy benefits and costs incurred for policies ceded. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387 + Details
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