Income Taxes (Significant components of the Company's deferred tax assets and liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets:
Property and casualty claim and claim adjustment expense reserves	$ 74	$ 125
Unearned premium reserves	142	206
Receivables	15	27
Employee benefits	154	272
Life settlement contracts		56
Deferred retroactive reinsurance benefit	68	117
Investment valuation differences	0	0
Other assets	105	148
Gross deferred tax assets	558	951
Deferred Tax Liabilities:
Investment valuation differences	55	57
Deferred acquisition costs	77	120
Net unrealized gains	233	309
Other liabilities	56	86
Gross deferred tax liabilities	421	572
Net deferred tax asset	$ 137	$ 379