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Reserves for Losses and Adjustment Expenses - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Insurance [Abstract] |  |  |
Provision for losses and LAE for claims incurred | $ (111.1) | $ (105.4) | $ (129.3)
Liability for unpaid claims and claims adjustment expense amount ceded |  | (125.5) | $ 85.8
Unpaid claims and claims adjustment expense assumed |  | $ 5.7 |

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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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- Definition Unpaid claims and claims adjustment expense specialty reinsurance contracts. + References No definition available. + Details
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Namespace Prefix: | ahl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration