Income Taxes - Deferred tax assets (liabilities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets
Loss reserves and tax discount	$ 104,000,000	$ 262,000,000
Unearned premium reserve and other underwriting related reserves	352,000,000	384,000,000
Investment-related items	194,000,000	458,000,000
Employee benefits	313,000,000	508,000,000
Alternative minimum tax credit	0	640,000,000
General business credit carryover	3,000,000	99,000,000
Net operating loss carryover	710,000,000	1,458,000,000
Foreign tax credit carryover	26,000,000	56,000,000
Other	1,000,000	0
Total Deferred Tax Assets	1,703,000,000	3,865,000,000
Deferred Tax Liabilities
Deferred acquisition costs	(103,000,000)	(176,000,000)
Net unrealized gains on investments	(306,000,000)	(348,000,000)
Other depreciable and amortizable assets	(130,000,000)	(243,000,000)
Other	0	(99,000,000)
Total Deferred Tax Liabilities	(539,000,000)	(866,000,000)
Net Deferred Tax Asset	1,164,000,000	$ 2,999,000,000
Alternative minimum tax credits reclassified to income tax receivable	$ 790,000,000