Fair Value Measurements - Fair Values of Note Payable and Long-Term Debt (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of note payable	$ 38	$ 32
Total	$ 1,095	$ 975
Senior Debentures 6.900 % Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Interest rate	6.90%	6.90%
Debt instrument, year of maturity	2028	2028
Fair value of senior debt	$ 35	$ 32
Senior Debentures 6.920 % Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Interest rate	6.92%	6.92%
Debt instrument, year of maturity	2028	2028
Fair value of senior debt	$ 512	$ 471
Senior Notes 6.125 % Due 2034
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Interest rate	6.125%	6.125%
Debt instrument, year of maturity	2034	2034
Fair value of senior debt	$ 510	$ 440
Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of note payable	0	0
Total	0	0
Quoted prices in active markets for identical assets (Level 1) \| Senior Debentures 6.900 % Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	0	0
Quoted prices in active markets for identical assets (Level 1) \| Senior Debentures 6.920 % Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	0	0
Quoted prices in active markets for identical assets (Level 1) \| Senior Notes 6.125 % Due 2034
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	0	0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of note payable	38	32
Total	1,095	975
Significant other observable inputs (Level 2) \| Senior Debentures 6.900 % Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	35	32
Significant other observable inputs (Level 2) \| Senior Debentures 6.920 % Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	512	471
Significant other observable inputs (Level 2) \| Senior Notes 6.125 % Due 2034
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	510	440
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of note payable	0	0
Total	0	0
Significant unobservable inputs (Level 3) \| Senior Debentures 6.900 % Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	0	0
Significant unobservable inputs (Level 3) \| Senior Debentures 6.920 % Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	0	0
Significant unobservable inputs (Level 3) \| Senior Notes 6.125 % Due 2034
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of senior debt	$ 0	$ 0

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of notes payable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20,22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_NotesPayableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=cinf_SeniorDebentures69PercentDue2028Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cinf_SeniorDebentures692PercentDue2028Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cinf_SeniorNotes6125PercentDue2034Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: