Summary of Investments Other Than Investments In Related Parties (Details) $ in Millions	Dec. 31, 2019 USD ($)
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	$ 14,985
Fair value	0
Balance sheet	19,746
Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	11,108
Fair value	11,698
Balance sheet	11,698
Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3,581
Fair value	7,752
Balance sheet	7,752
Other than Securities Investment
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	296
Fair value	0
Balance sheet	296
States, Municipalities and Political Subdivisions | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	4,477
Fair value	4,728
Balance sheet	4,728
States, Municipalities and Political Subdivisions | Fixed Maturities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	38
Fair value	38
Balance sheet	38
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	3,320
Fair value	3,515
Balance sheet	3,515
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	199
Fair value	210
Balance sheet	210
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	46
Fair value	48
Balance sheet	48
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	525
Fair value	556
Balance sheet	556
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	348
Fair value	360
Balance sheet	360
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
United States Government | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	102
Fair value	104
Balance sheet	104
United States Government | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	65
Fair value	67
Balance sheet	67
United States Government | Fixed Maturities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
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
United States Government | Fixed Maturities | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	35
Fair value	35
Balance sheet	35
Government-Sponsored Enterprises | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	137
Fair value	136
Balance sheet	136
Government-Sponsored Enterprises | Fixed Maturities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	10
Fair value	10
Balance sheet	10
Government-Sponsored Enterprises | Fixed Maturities | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5
Fair value	5
Balance sheet	5
Government-Sponsored Enterprises | Fixed Maturities | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	122
Fair value	121
Balance sheet	121
Foreign Government | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	28
Fair value	28
Balance sheet	28
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
Foreign Government | Fixed Maturities | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	18
Fair value	18
Balance sheet	18
All Other Corporate Bonds | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	6,364
Fair value	6,702
Balance sheet	6,702
All Other Corporate Bonds | Fixed Maturities | 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 | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	2,956
Fair value	3,095
Balance sheet	3,095
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	107
Fair value	112
Balance sheet	112
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	33
Fair value	34
Balance sheet	34
All Other Corporate Bonds | Fixed Maturities | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	58
Fair value	58
Balance sheet	58
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	175
Fair value	186
Balance sheet	186
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	3,033
Fair value	3,215
Balance sheet	3,215
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
Common Stock | Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3,371
Fair value	7,518
Balance sheet	7,518
Common Stock | Equity Securities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1,426
Fair value	3,068
Balance sheet	3,068
Common Stock | Equity Securities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1,763
Fair value	4,063
Balance sheet	4,063
Common Stock | Equity Securities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	58
Fair value	141
Balance sheet	141
Common Stock | Equity Securities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	17
Fair value	33
Balance sheet	33
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	90
Fair value	184
Balance sheet	184
Common Stock | Equity Securities | CSU Producers Resources Inc.
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	17
Fair value	29
Balance sheet	29
Nonredeemable Preferred Stock | Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	210
Fair value	234
Balance sheet	234
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
Nonredeemable Preferred Stock | Equity Securities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	197
Fair value	218
Balance sheet	218
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	12
Fair value	15
Balance sheet	15
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	32
Fair value	0
Balance sheet	32
Lloyd's and other deposits | Other than Securities Investment | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	67
Fair value	0
Balance sheet	67
Lloyd's and other deposits | Other than Securities Investment | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	97
Fair value	0
Balance sheet	97
Limited Partnership | Other than Securities Investment | Cincinnati Financial Corporation
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 | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	37
Balance sheet	37
Limited Partnership | Other than Securities Investment | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3
Balance sheet	3
Real Estate | Other than Securities Investment | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5
Balance sheet	5
Real Estate | Other than Securities Investment | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	8
Balance sheet	8
Real Estate | Other than Securities Investment | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	16
Balance sheet	$ 16