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Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Insurance [Abstract] |  |  |
Reserve releases | $ (99.7) | $ (92.8) | $ (105.4)
Amount of reserves ceded |  |  | $ 125.5

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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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Namespace Prefix: | ahl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_InsuranceAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration