Investments (Details) - Unrealized Losses - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 2,900	$ 19,036
Continuous unrealized loss position, less than 12 months, gross unrealized losses	19	399
Continuous unrealized loss position, 12 months or longer, fair value	1,145	12,579
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	10	450
Continuous unrealized loss position, total, fair value	4,045	31,615
Continuous unrealized loss position, total, gross unrealized losses	29	849
Unrealized losses for fixed maturity investments for which fair value is less than 80% of amortized cost, total	0
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	5	484
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0	5
Continuous unrealized loss position, 12 months or longer, fair value	193	1,011
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	11
Continuous unrealized loss position, total, fair value	198	1,495
Continuous unrealized loss position, total, gross unrealized losses	0	16
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	668	5,241
Continuous unrealized loss position, less than 12 months, gross unrealized losses	6	82
Continuous unrealized loss position, 12 months or longer, fair value	12	3,298
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	125
Continuous unrealized loss position, total, fair value	680	8,539
Continuous unrealized loss position, total, gross unrealized losses	6	207
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	257	96
Continuous unrealized loss position, less than 12 months, gross unrealized losses	1	0
Continuous unrealized loss position, 12 months or longer, fair value	147	328
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	1	5
Continuous unrealized loss position, total, fair value	404	424
Continuous unrealized loss position, total, gross unrealized losses	2	5
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	399	593
Continuous unrealized loss position, less than 12 months, gross unrealized losses	2	9
Continuous unrealized loss position, 12 months or longer, fair value	131	1,070
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	1	29
Continuous unrealized loss position, total, fair value	530	1,663
Continuous unrealized loss position, total, gross unrealized losses	3	38
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	1,571	12,622
Continuous unrealized loss position, less than 12 months, gross unrealized losses	10	303
Continuous unrealized loss position, 12 months or longer, fair value	662	6,872
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	8	280
Continuous unrealized loss position, total, fair value	2,233	19,494
Continuous unrealized loss position, total, gross unrealized losses	$ 18	$ 583