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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

X

- 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

X

- References

No definition available.

+ Details

Name:	us-gaap_InsuranceAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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
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