Investments (Details) - Investment Information, Losses - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 3,040	$ 2,900
Continuous unrealized loss position, less than 12 months, gross unrealized losses	42	19
Continuous unrealized loss position, 12 months or longer, fair value	121	1,145
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	7	10
Continuous unrealized loss position, total, fair value	3,161	4,045
Continuous unrealized loss position, total, gross unrealized losses	49	29
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	169	5
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0	0
Continuous unrealized loss position, 12 months or longer, fair value	0	193
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	0
Continuous unrealized loss position, total, fair value	169	198
Continuous unrealized loss position, total, gross unrealized losses	0	0
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	1,751	668
Continuous unrealized loss position, less than 12 months, gross unrealized losses	14	6
Continuous unrealized loss position, 12 months or longer, fair value	0	12
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	0
Continuous unrealized loss position, total, fair value	1,751	680
Continuous unrealized loss position, total, gross unrealized losses	14	6
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	14	257
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0	1
Continuous unrealized loss position, 12 months or longer, fair value	0	147
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	1
Continuous unrealized loss position, total, fair value	14	404
Continuous unrealized loss position, total, gross unrealized losses	0	2
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	11	399
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0	2
Continuous unrealized loss position, 12 months or longer, fair value	1	131
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	1
Continuous unrealized loss position, total, fair value	12	530
Continuous unrealized loss position, total, gross unrealized losses	0	3
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	1,095	1,571
Continuous unrealized loss position, less than 12 months, gross unrealized losses	28	10
Continuous unrealized loss position, 12 months or longer, fair value	120	662
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	7	8
Continuous unrealized loss position, total, fair value	1,215	2,233
Continuous unrealized loss position, total, gross unrealized losses	$ 35	$ 18