Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Income Tax Contingency
Tax at statutory rate, percentage	21.00%	21.00%	21.00%	21.00%
Unrecognized Tax Benefits	$ 34		$ 34		$ 34
Deferred tax assets valuation allowance	0		0		0
Cincinnati Global
Income Tax Contingency
Change in deferred tax assets	4		0
Change in deferred tax valuation allowance	4		0
Net deferred tax assets	56		56
Deferred tax assets valuation allowance	56		56
Cincinnati Global | United States
Income Tax Contingency
Operating loss carryforwards	26		26		26
Cincinnati Global | United Kingdom
Income Tax Contingency
Operating loss carryforwards	$ 128		$ 128		$ 108