Income Taxes (Significant components of the Company's deferred tax assets and liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Deferred Tax Assets:
Property and casualty claim and claim adjustment expense reserves	$ 108		$ 74
Unearned premium reserves	108		142
Receivables	15		15
Employee benefits	143		154
Deferred retroactive reinsurance benefit	79		68
Other assets	131		105
Gross deferred tax assets	584		558
Deferred Tax Liabilities:
Investment valuation differences	44		55
Deferred acquisition costs	78		77
Net unrealized gains	14		233
Software and hardware	44		46
Other liabilities	12		10
Gross deferred tax liabilities	192		421
Net deferred tax asset	$ 392	$ 158	$ 137