Schedule VI. Supplemental Information Concerning Property and Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Incurred claim and claim adjustment expenses related to current year	$ 5,201	$ 5,025	$ 4,934
Incurred claim and claim adjustment expenses related to prior years	(381)	(342)	(255)
Consolidated Property and Casualty Insurance Entity
Deferred acquisition costs	632	599
Reserves for unpaid claim and claim adjustment expenses	22,004	22,343
Discount deducted from claim and claim adjustment expense reserves above (based on interest rates ranging from 3.5% to 8.0%)	1,434	1,572
Unearned premiums	4,029	3,762
Net written premiums	7,069	6,988	6,962
Net earned premiums	6,988	6,924	6,921
Net investment income	1,992	1,952	1,807
Incurred claim and claim adjustment expenses related to current year		5,025	4,934
Incurred claim and claim adjustment expenses related to prior years		(342)	(255)
Amortization of deferred acquisition costs	1,233	1,235	1,540
Paid claim and claim adjustment expenses	$ 5,341	$ 5,134	$ 4,945
Consolidated Property and Casualty Insurance Entity | Minimum
Supplemental Information Parenthetical
Supplemental Information for Property, Casualty Insurance Underwriters, Interest Rate at which Discount Computed	3.50%	3.50%
Consolidated Property and Casualty Insurance Entity | Maximum
Supplemental Information Parenthetical
Supplemental Information for Property, Casualty Insurance Underwriters, Interest Rate at which Discount Computed	8.00%	8.00%