Statutory Accounting Information (Schedule Of Statutory Net Income And Statutory Surplus (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
The Cincinnati Insurance Company
Statutory Accounting Practices
SAP net income (loss)	$ 401	$ 434	$ 534
Capital and surplus	5,094	4,686
The Cincinnati Casualty Company
Statutory Accounting Practices
SAP net income (loss)	21	11	12
Capital and surplus	392	360
The Cincinnati Indemnity Company
Statutory Accounting Practices
SAP net income (loss)	4	4	3
Capital and surplus	100	93
The Cincinnati Specialty Underwriters Insurance Company
Statutory Accounting Practices
SAP net income (loss)	58	57	49
Capital and surplus	436	372
The Cincinnati Life Insurance Company
Statutory Accounting Practices
SAP net income (loss)	12	2	$ (11)
Capital and surplus	$ 195	$ 200