| **Accumulated Other Comprehensive Income (Tables)**       | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             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| **Accumulated Other Comprehensive Income [Abstract]**     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       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| Schedule of Accumulated Other Comprehensive Income (Loss) | (Dollars in millions) Three months ended June 30,  2015  2014  Before tax Income tax Net  Before tax Income tax NetInvestments:             AOCI, beginning of period $2,676 $927 $1,749  $2,452 $849 $1,603OCI before realized gains recognized in net income (260) (91) (169)  252 87 165Realized gains recognized in net income (59) (21) (38)  (17) (5) (12)OCI (319) (112) (207)  235 82 153AOCI, end of period $2,357 $815 $1,542  $2,687 $931 $1,756              Pension obligations:             AOCI, beginning of period $(35) $(12) $(23)  $(19) $(6) $(13)OCI excluding amortization recognized in net income — — —  — — —Amortization recognized in net income 2 1 1  — — —OCI 2 1 1  — — —AOCI, end of period $(33) $(11) $(22)  $(19) $(6) $(13)              Life deferred acquisition costs, life policy reserves and other:             AOCI, beginning of period $(14) $(4) $(10)  $(17) $(5) $(12)OCI before realized gains recognized in net income 7 3 4  (2) — (2)Realized gains recognized in net income (1) (1) —  3 — 3OCI 6 2 4  1 — 1AOCI, end of period $(8) $(2) $(6)  $(16) $(5) $(11)              Summary of AOCI:             AOCI, beginning of period $2,627 $911 $1,716  $2,416 $838 $1,578Investments OCI (319) (112) (207)  235 82 153Pension obligations OCI 2 1 1  — — —Life deferred acquisition costs, life policy reserves and other OCI 6 2 4  1 — 1Total OCI (311) (109) (202)  236 82 154AOCI, end of period $2,316 $802 $1,514  $2,652 $920 $1,732              (Dollars in millions)Six months ended June 30, 2015  2014 Before tax Income tax Net  Before tax Income tax NetInvestments:            AOCI, beginning of period$2,719 $942 $1,777  $2,335 $808 $1,527OCI excluding realized gains recognized in net income(257) (90) (167)  387 135 252Realized gains recognized in net income(105) (37) (68)  (35) (12) (23)OCI(362) (127) (235)  352 123 229AOCI, end of period$2,357 $815 $1,542  $2,687 $931 $1,756             Pension obligations:            AOCI, beginning of period$(36) $(12) $(24)  $(18) $(6) $(12)OCI excluding amortization recognized in net income— — —  — — —Amortization recognized in net income3 1 2  (1) — (1)OCI3 1 2  (1) — (1)AOCI, end of period$(33) $(11) $(22)  $(19) $(6) $(13)             Life deferred acquisition costs, life policy reserves and other:            AOCI, beginning of period$(12) $(3) $(9)  $(16) $(5) $(11)OCI excluding realized gains recognized in net income6 2 4  1 1 —Realized gains recognized in net income(2) (1) (1)  (1) (1) —OCI4 1 3  — — —AOCI, end of period$(8) $(2) $(6)  $(16) $(5) $(11)             Summary of AOCI:            AOCI, beginning of period$2,671 $927 $1,744  $2,301 $797 $1,504Investments OCI(362) (127) (235)  352 123 229Pension obligations OCI3 1 2  (1) — (1)Life deferred acquisition costs, life policy reserves and other OCI4 1 3  — — —Total OCI(355) (125) (230)  351 123 228AOCI, end of period$2,316 $802 $1,514  $2,652 $920 $1,732              |

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| - Definition
Accumulated Other Comprehensive Income [Abstract]
+ References
No definition available.
+ Details**Name:**cinf_AccumulatedOtherComprehensiveIncomeAbstract**Namespace Prefix:**cinf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the components of accumulated other comprehensive income (loss).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580
+ Details**Name:**us-gaap_ScheduleOfAccumulatedOtherComprehensiveIncomeLossTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |