Income Taxes (Significant components of the Company's deferred tax assets and liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets:
Property and casualty claim and claim adjustment expense reserves	$ 157	$ 129
Unearned premium reserves	174	153
Receivables	11	11
Employee benefits	122	127
Deferred retroactive reinsurance benefit	83	82
Other assets	143	132
Gross deferred tax assets	690	634
Deferred Tax Liabilities:
Investment valuation differences	28	40
Deferred acquisition costs	93	83
Net unrealized gains	453	264
Software and hardware	31	34
Other liabilities	19	14
Gross deferred tax liabilities	624	435
Net deferred tax asset	$ 66	$ 199