Investments (Details) - Investment Information, Losses - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 30,578		$ 30,578		$ 9,132
Continuous unrealized loss position, less than 12 months, gross unrealized losses	670		670		68
Continuous unrealized loss position, 12 months or longer, fair value	8,505		8,505		5,780
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	426		426		110
Continuous unrealized loss position, total, fair value	39,083		39,083		14,912
Continuous unrealized loss position, total, gross unrealized losses	1,096		1,096		178
Impairment charges	0		1
Cumulative OTTI credit losses recognized for securities held	$ 54	$ 82	$ 54	$ 82
Cumulative credit component of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income (loss) for fixed maturities held, as a percentage of the fixed maturity portfolio on a pre-tax basis (less than)	1.00%	1.00%	1.00%	1.00%
Cumulative credit component of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income (loss) for fixed maturities held, as a percentage of shareholders' equity on an after-tax basis (less than)	1.00%	1.00%	1.00%	1.00%
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value					9,153
Continuous unrealized loss position, less than 12 months, gross unrealized losses					68
Continuous unrealized loss position, 12 months or longer, fair value					5,870
Continuous unrealized loss position, 12 months or longer, gross unrealized losses					117
Continuous unrealized loss position, total, fair value					15,023
Continuous unrealized loss position, total, gross unrealized losses					185
Impairment charges		$ 5		$ 12
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 1,241		$ 1,241		1,150
Continuous unrealized loss position, less than 12 months, gross unrealized losses	20		20		5
Continuous unrealized loss position, 12 months or longer, fair value	625		625		470
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	9		9		3
Continuous unrealized loss position, total, fair value	1,866		1,866		1,620
Continuous unrealized loss position, total, gross unrealized losses	29		29		8
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	10,078		10,078		505
Continuous unrealized loss position, less than 12 months, gross unrealized losses	216		216		2
Continuous unrealized loss position, 12 months or longer, fair value	3,063		3,063		2,959
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	186		186		47
Continuous unrealized loss position, total, fair value	13,141		13,141		3,464
Continuous unrealized loss position, total, gross unrealized losses	402		402		49
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	504		504		394
Continuous unrealized loss position, less than 12 months, gross unrealized losses	3		3		6
Continuous unrealized loss position, 12 months or longer, fair value	294		294		111
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	8		8		4
Continuous unrealized loss position, total, fair value	798		798		505
Continuous unrealized loss position, total, gross unrealized losses	11		11		10
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	1,265		1,265		1,021
Continuous unrealized loss position, less than 12 months, gross unrealized losses	33		33		7
Continuous unrealized loss position, 12 months or longer, fair value	661		661		250
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	29		29		4
Continuous unrealized loss position, total, fair value	1,926		1,926		1,271
Continuous unrealized loss position, total, gross unrealized losses	62		62		11
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	17,490		17,490		6,062
Continuous unrealized loss position, less than 12 months, gross unrealized losses	398		398		48
Continuous unrealized loss position, 12 months or longer, fair value	3,862		3,862		1,990
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	194		194		52
Continuous unrealized loss position, total, fair value	21,352		21,352		8,052
Continuous unrealized loss position, total, gross unrealized losses	$ 592		$ 592		100
Equity securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value					21
Continuous unrealized loss position, 12 months or longer, fair value					90
Continuous unrealized loss position, 12 months or longer, gross unrealized losses					7
Continuous unrealized loss position, total, fair value					111
Continuous unrealized loss position, total, gross unrealized losses					7
Public common stock [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value					18
Continuous unrealized loss position, 12 months or longer, fair value					34
Continuous unrealized loss position, 12 months or longer, gross unrealized losses					1
Continuous unrealized loss position, total, fair value					52
Continuous unrealized loss position, total, gross unrealized losses					1
Non-redeemable preferred stock [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value					3
Continuous unrealized loss position, 12 months or longer, fair value					56
Continuous unrealized loss position, 12 months or longer, gross unrealized losses					6
Continuous unrealized loss position, total, fair value					59
Continuous unrealized loss position, total, gross unrealized losses					$ 6