Summary of Investments - Other than Investments in Related Parties (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	$ 10,312
Balance sheet	11,801
Debt Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	8,084
Fair value	8,779
Balance sheet	8,779
Debt Securities | US Government Agencies Debt Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	6
Fair value	7
Balance sheet	7
Debt Securities | US Government Agencies Debt Securities | Cincinnati Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	1
Fair value	1
Balance sheet	1
Debt Securities | US Government Agencies Debt Securities | Cincinnati Casualty Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	1
Fair value	1
Balance sheet	1
Debt Securities | US Government Agencies Debt Securities | Cincinnati Indemnity Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	1
Fair value	1
Balance sheet	1
Debt Securities | US Government Agencies Debt Securities | Cincinnati Life Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	3
Fair value	4
Balance sheet	4
Debt Securities | US Government-sponsored Enterprises Debt Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	159
Fair value	160
Balance sheet	160
Debt Securities | US Government-sponsored Enterprises Debt Securities | Cincinnati Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	5
Fair value	5
Balance sheet	5
Debt Securities | US Government-sponsored Enterprises Debt Securities | Cincinnati Life Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	154
Fair value	155
Balance sheet	155
Debt Securities | Foreign Government
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	3
Fair value	3
Balance sheet	3
Debt Securities | Foreign Government | Cincinnati Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	3
Fair value	3
Balance sheet	3
Debt Securities | US States and Political Subdivisions Debt Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	3,006
Fair value	3,252
Balance sheet	3,252
Debt Securities | US States and Political Subdivisions Debt Securities | Cincinnati Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,497
Fair value	2,694
Balance sheet	2,694
Debt Securities | US States and Political Subdivisions Debt Securities | Cincinnati Casualty Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	138
Fair value	148
Balance sheet	148
Debt Securities | US States and Political Subdivisions Debt Securities | Cincinnati Indemnity Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	36
Fair value	40
Balance sheet	40
Debt Securities | US States and Political Subdivisions Debt Securities | Cincinnati Specialty Underwriters Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	130
Fair value	144
Balance sheet	144
Debt Securities | US States and Political Subdivisions Debt Securities | Cincinnati Life Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	204
Fair value	225
Balance sheet	225
Debt Securities | US States and Political Subdivisions Debt Securities | CSU Producers Resources Inc.
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	1
Fair value	1
Balance sheet	1
Debt Securities | Convertibles and Bonds with Warrants Attached
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	59
Fair value	59
Balance sheet	59
Debt Securities | Convertibles and Bonds with Warrants Attached | Cincinnati Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	50
Fair value	50
Balance sheet	50
Debt Securities | Convertibles and Bonds with Warrants Attached | Cincinnati Life Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	4
Fair value	4
Balance sheet	4
Debt Securities | Convertibles and Bonds with Warrants Attached | Cincinnati Financial Corporation
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	5
Fair value	5
Balance sheet	5
Debt Securities | All Other Corporate Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	4,851
Fair value	5,298
Balance sheet	5,298
Debt Securities | All Other Corporate Bonds | Cincinnati Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,380
Fair value	2,602
Balance sheet	2,602
Debt Securities | All Other Corporate Bonds | Cincinnati Casualty Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	80
Fair value	85
Balance sheet	85
Debt Securities | All Other Corporate Bonds | Cincinnati Indemnity Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	20
Fair value	22
Balance sheet	22
Debt Securities | All Other Corporate Bonds | Cincinnati Specialty Underwriters Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	114
Fair value	121
Balance sheet	121
Debt Securities | All Other Corporate Bonds | Cincinnati Life Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,061
Fair value	2,241
Balance sheet	2,241
Debt Securities | All Other Corporate Bonds | CSU Producers Resources Inc.
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	7
Fair value	7
Balance sheet	7
Debt Securities | All Other Corporate Bonds | Cincinnati Financial Corporation
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	189
Fair value	220
Balance sheet	220
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,162
Fair value	2,956
Balance sheet	2,956
Equity Securities | Common Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,088
Fair value	2,854
Balance sheet	2,854
Equity Securities | Common Stock | Cincinnati Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	1,371
Fair value	1,920
Balance sheet	1,920
Equity Securities | Common Stock | Cincinnati Casualty Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	46
Fair value	62
Balance sheet	62
Equity Securities | Common Stock | Cincinnati Indemnity Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	13
Fair value	16
Balance sheet	16
Equity Securities | Common Stock | Cincinnati Specialty Underwriters Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	41
Fair value	45
Balance sheet	45
Equity Securities | Common Stock | Cincinnati Life Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	3
Fair value	3
Balance sheet	3
Equity Securities | Common Stock | CSU Producers Resources Inc.
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2
Fair value	2
Balance sheet	2
Equity Securities | Common Stock | Cincinnati Financial Corporation
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	612
Fair value	806
Balance sheet	806
Equity Securities | Nonredeemable Preferred Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	74
Fair value	102
Balance sheet	102
Equity Securities | Nonredeemable Preferred Stock | Cincinnati Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	67
Fair value	89
Balance sheet	89
Equity Securities | Nonredeemable Preferred Stock | Cincinnati Life Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	7
Fair value	13
Balance sheet	13
Other than Securities Investment
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	66
Balance sheet	66
Other than Securities Investment | Policy Loans | Cincinnati Life Insurance Company
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	37
Balance sheet	37
Other than Securities Investment | Limited Partnership | Cincinnati Financial Corporation
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	29
Balance sheet	$ 29