Condensed Financial Statements Of Parent Company Condensed Statements Of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions
Net Income	$ 642	$ 102	$ 100	$ 201	$ 100	$ 180	$ 123	$ 188	$ 1,045	$ 591	$ 634
Unrealized gains and (losses) on investments available for sale									598	345	(405)
Unrealized gains and (losses) on investments available for sale, investments held by subsidiaries and other									914	521	(612)
Amortization of pension actuarial gains (losses) and prior service cost									14	16	(6)
OCI									928	537	(618)
Income taxes on above of other comprehensive income (loss)									325	188	(218)
Other comprehensive income (loss), net of tax									603	349	(400)
Comprehensive Income									1,648	940	234
Cincinnati Financial Corporation
Condensed Financial Statements, Captions
Net Income									1,045	591	634
Unrealized gains and (losses) on investments available for sale									391	221	(111)
Unrealized gains and (losses) on investments held by subsidiaries									672	434	(444)
Reclassification adjustment for (gains) and losses included in net income									(28)	(27)	19
Reclassification adjustment for (gains) included in net income on subsidiaries									(120)	(97)	(89)
Unrealized (losses) and gains on other									(2)	(6)	0
Unrealized gains and (losses) on other subsidiaries									$ 1	$ (4)	$ 13