Statutory Net Income and Statutory Surplus (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cincinnati Insurance Company
Statutory Accounting Practices [Line Items]
SAP Net Income (Loss)	$ 120	$ 318	$ 339
Capital and Surplus	3,747	3,777
Cincinnati Casualty Company
Statutory Accounting Practices [Line Items]
SAP Net Income (Loss)	15	10	29
Capital and Surplus	280	269
Cincinnati Indemnity Company
Statutory Accounting Practices [Line Items]
SAP Net Income (Loss)	2	2	8
Capital and Surplus	73	70
Cincinnati Specialty Underwriters Insurance Company
Statutory Accounting Practices [Line Items]
SAP Net Income (Loss)	11	1	(7)
Capital and Surplus	186	172
Cincinnati Life Insurance Company
Statutory Accounting Practices [Line Items]
SAP Net Income (Loss)	(13)	15	15
Capital and Surplus	$ 281	$ 303