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Reinsurance - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Reinsurance Disclosures [Abstract] |  |  |
Liability for unpaid claims and claims adjustment expense amount ceded | $ 125.5 | $ 85.8 | $ 0.0

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- Definition Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed of or Ceded + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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