Summary of Investments Other Than Investments In Related Parties (Details)1 $ in Millions	Dec. 31, 2016 USD ($)
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	$ 12,875
Fair value	0
Balance sheet	15,500
Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	9,799
Fair value	10,085
Balance sheet	10,085
Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	2,995
Fair value	5,334
Balance sheet	5,334
Other than Securities Investment
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	81
Fair value	0
Balance sheet	81
States, Municipalities and Political Subdivisions | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3,770
Fair value	3,828
Balance sheet	3,828
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	2,900
Fair value	2,934
Balance sheet	2,934
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	149
Fair value	150
Balance sheet	150
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	39
Fair value	40
Balance sheet	40
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	272
Fair value	287
Balance sheet	287
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Specialty Underwriters Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	409
Fair value	416
Balance sheet	416
States, Municipalities and Political Subdivisions | Fixed Maturities | CSU Producers Resources Inc.
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
Convertibles and Bonds with Warrants Attached | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5
Fair value	5
Balance sheet	5
Convertibles and Bonds with Warrants Attached | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5
Fair value	5
Balance sheet	5
United States Government | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	10
Fair value	10
Balance sheet	10
United States Government | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	7
Fair value	7
Balance sheet	7
United States Government | Fixed Maturities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	2
Fair value	2
Balance sheet	2
United States Government | Fixed Maturities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
Government-Sponsored Enterprises | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	167
Fair value	164
Balance sheet	164
Government-Sponsored Enterprises | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	167
Fair value	164
Balance sheet	164
Foreign Government | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	10
Fair value	10
Balance sheet	10
Foreign Government | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	10
Fair value	10
Balance sheet	10
Common Stock | Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	2,812
Fair value	5,123
Balance sheet	5,123
Common Stock | Equity Securities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1,600
Fair value	3,053
Balance sheet	3,053
Common Stock | Equity Securities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	50
Fair value	105
Balance sheet	105
Common Stock | Equity Securities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	13
Fair value	24
Balance sheet	24
Common Stock | Equity Securities | The Cincinnati Specialty Underwriters Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	51
Fair value	91
Balance sheet	91
Common Stock | Equity Securities | CSU Producers Resources Inc.
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	15
Fair value	18
Balance sheet	18
Common Stock | Equity Securities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1,083
Fair value	1,832
Balance sheet	1,832
Nonredeemable Preferred Stock | Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	183
Fair value	211
Balance sheet	211
Nonredeemable Preferred Stock | Equity Securities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	177
Fair value	201
Balance sheet	201
Nonredeemable Preferred Stock | Equity Securities | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5
Fair value	9
Balance sheet	9
Nonredeemable Preferred Stock | Equity Securities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
All Other Corporate Bonds | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5,837
Fair value	6,068
Balance sheet	6,068
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	2,727
Fair value	2,836
Balance sheet	2,836
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	118
Fair value	123
Balance sheet	123
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	30
Fair value	31
Balance sheet	31
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	2,760
Fair value	2,865
Balance sheet	2,865
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Specialty Underwriters Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	155
Fair value	160
Balance sheet	160
All Other Corporate Bonds | Fixed Maturities | CSU Producers Resources Inc.
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
All Other Corporate Bonds | Fixed Maturities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	46
Fair value	52
Balance sheet	52
Policy Loans | Other than Securities Investment | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	31
Fair value	0
Balance sheet	31
Limited Partnership | Other than Securities Investment | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	23
Fair value	0
Balance sheet	23
Real Estate [Member] | Other than Securities Investment | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	21
Balance sheet	21
Real Estate [Member] | Other than Securities Investment | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	6
Balance sheet	$ 6