Segment Information - Additional information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Dec. 31, 2011
Personal Lines
Segment Reporting, Other Significant Reconciling Item [Line Items]
Adjusting and other expense between segments	$ 23
Commercial Lines
Segment Reporting, Other Significant Reconciling Item [Line Items]
Adjusting and other expense between segments	$ (23)

The amount of allocation of the adjusting and other expense portion of loss adjustment expense between the commercial and personal lines segments.

No definition available.

Name:	cinf_AdjustingAndOtherExpenseAllocationBetweenSegments
Namespace Prefix:	cinf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration