Income Taxes (Significant components of the Company's deferred tax assets and liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets:
Property and casualty claim and claim adjustment expense reserves	$ 125	$ 178
Unearned premium reserves	206	230
Receivables	27	29
Employee benefits	272	281
Life settlement contracts	56	48
Deferred retroactive reinsurance benefit	117	84
Investment valuation differences	0	29
Other assets	148	142
Gross deferred tax assets	951	1,021
Deferred Tax Liabilities:
Investment valuation differences	57	0
Deferred acquisition costs	120	117
Net unrealized gains	309	202
Other liabilities	86	64
Deferred Tax Liabilities, Gross	572	383
Net deferred tax asset (liability)	$ 379	$ 638