| **Accumulated Other Comprehensive Income (Tables)**       | 9 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 September 30,  2016  2015  Before tax Income tax Net  Before tax Income tax NetInvestments:             AOCI, beginning of period $2,673 $925 $1,748  $2,357 $815 $1,542OCI before realized gains recognized in net income 119 41 78  (363) (126) (237)Realized gains recognized in net income (57) (20) (37)  (2) (1) (1)OCI 62 21 41  (365) (127) (238)AOCI, end of period $2,735 $946 $1,789  $1,992 $688 $1,304              Pension obligations:             AOCI, beginning of period $(40) $(13) $(27)  $(33) $(11) $(22)OCI excluding amortization recognized in net income — — —  — — —Amortization recognized in net income — — —  1 — 1OCI — — —  1 — 1AOCI, end of period $(40) $(13) $(27)  $(32) $(11) $(21)              Life deferred acquisition costs, life policy reserves and other:             AOCI, beginning of period $(10) $(3) $(7)  $(8) $(2) $(6)OCI before realized gains recognized in net income (4) (1) (3)  5 1 4Realized gains recognized in net income 1 1 —  (1) — (1)OCI (3) — (3)  4 1 3AOCI, end of period $(13) $(3) $(10)  $(4) $(1) $(3)              Summary of AOCI:             AOCI, beginning of period $2,623 $909 $1,714  $2,316 $802 $1,514Investments OCI 62 21 41  (365) (127) (238)Pension obligations OCI — — —  1 — 1Life deferred acquisition costs, life policy reserves and other OCI (3) — (3)  4 1 3Total OCI 59 21 38  (360) (126) (234)AOCI, end of period $2,682 $930 $1,752  $1,956 $676 $1,280              (Dollars in millions)Nine months ended September 30, 2016  2015 Before tax Income tax Net  Before tax Income tax NetInvestments:            AOCI, beginning of period$2,094 $722 $1,372  $2,719 $942 $1,777OCI excluding realized gains recognized in net income801 280 521  (620) (216) (404)Realized gains recognized in net income(160) (56) (104)  (107) (38) (69)OCI641 224 417  (727) (254) (473)AOCI, end of period$2,735 $946 $1,789  $1,992 $688 $1,304             Pension obligations:            AOCI, beginning of period$(42) $(14) $(28)  $(36) $(12) $(24)OCI excluding amortization recognized in net income— — —  — — —Amortization recognized in net income2 1 1  4 1 3OCI2 1 1  4 1 3AOCI, end of period$(40) $(13) $(27)  $(32) $(11) $(21)             Life deferred acquisition costs, life policy reserves and other:            AOCI, beginning of period$1 $1 $—  $(12) $(3) $(9)OCI excluding realized gains recognized in net income(13) (4) (9)  11 3 8Realized gains recognized in net income(1) — (1)  (3) (1) (2)OCI(14) (4) (10)  8 2 6AOCI, end of period$(13) $(3) $(10)  $(4) $(1) $(3)             Summary of AOCI:            AOCI, beginning of period$2,053 $709 $1,344  $2,671 $927 $1,744Investments OCI641 224 417  (727) (254) (473)Pension obligations OCI2 1 1  4 1 3Life deferred acquisition costs, life policy reserves and other OCI(14) (4) (10)  8 2 6Total OCI629 221 408  (715) (251) (464)AOCI, end of period$2,682 $930 $1,752  $1,956 $676 $1,280              |

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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=66004124&loc=SL7669686-108580
+ Details**Name:**us-gaap_ScheduleOfAccumulatedOtherComprehensiveIncomeLossTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |