Income Taxes (Significant components of the Company's deferred tax assets and liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018
Deferred Tax Assets:
Property and casualty claim and claim adjustment expense reserves	$ 129	$ 108
Unearned premium reserves	153	108
Receivables	11	15
Employee benefits	127	143
Deferred retroactive reinsurance benefit	82	79
Other assets	132	131
Gross deferred tax assets	634	584
Deferred Tax Liabilities:
Investment valuation differences	40	44
Deferred acquisition costs	83	78
Net unrealized gains	264	14
Software and hardware	34	44
Other liabilities	14	12
Gross deferred tax liabilities	435	192
Net deferred tax asset	$ 199	$ 392	$ 158