Summary of Investments Other Than Investments In Related Parties (Details)1 $ in Millions	Dec. 31, 2018 USD ($)
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	$ 14,134
Balance sheet	16,732
Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	10,643
Fair value	10,689
Balance sheet	10,689
Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3,368
Fair value	5,920
Balance sheet	5,920
Other than Securities Investment
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	123
Balance sheet	123
States, Municipalities and Political Subdivisions | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	4,251
Fair value	4,304
Balance sheet	4,304
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	19
Fair value	19
Balance sheet	19
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	3,245
Fair value	3,282
Balance sheet	3,282
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	181
Fair value	182
Balance sheet	182
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	44
Fair value	44
Balance sheet	44
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	302
Fair value	310
Balance sheet	310
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	459
Fair value	466
Balance sheet	466
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	67
Fair value	67
Balance sheet	67
United States Government | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	64
Fair value	64
Balance sheet	64
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	316
Fair value	310
Balance sheet	310
Government-Sponsored Enterprises | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5
Fair value	6
Balance sheet	6
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	311
Fair value	304
Balance sheet	304
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	3,195
Fair value	5,742
Balance sheet	5,742
Common Stock | Equity Securities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1,281
Fair value	2,233
Balance sheet	2,233
Common Stock | Equity Securities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1,731
Fair value	3,216
Balance sheet	3,216
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	108
Balance sheet	108
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	26
Balance sheet	26
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	91
Fair value	138
Balance sheet	138
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	21
Balance sheet	21
Nonredeemable Preferred Stock | Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	173
Fair value	178
Balance sheet	178
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	167
Fair value	169
Balance sheet	169
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	8
Balance sheet	8
All Other Corporate Bonds | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5,999
Fair value	5,998
Balance sheet	5,998
All Other Corporate Bonds | Fixed Maturities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	16
Fair value	16
Balance sheet	16
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	2,861
Fair value	2,860
Balance sheet	2,860
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	112
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	31
Fair value	32
Balance sheet	32
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	2,820
Fair value	2,820
Balance sheet	2,820
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	158
Fair value	157
Balance sheet	157
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
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	33
Fair value	0
Balance sheet	33
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	10
Balance sheet	9
Limited Partnership | Other than Securities Investment | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	30
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	20
Balance sheet	20
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
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	24
Balance sheet	$ 24