Investments (Details) - Investment Information, Losses - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 13,402	$ 1,045
Continuous unrealized loss position, less than 12 months, gross unrealized losses	254	8
Continuous unrealized loss position, 12 months or longer, fair value	313	98
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	12	3
Continuous unrealized loss position, total, fair value	13,715	1,143
Continuous unrealized loss position, total, gross unrealized losses	266	11
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	1,804	92
Continuous unrealized loss position, less than 12 months, gross unrealized losses	20	0
Continuous unrealized loss position, 12 months or longer, fair value	5	0
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	0
Continuous unrealized loss position, total, fair value	1,809	92
Continuous unrealized loss position, total, gross unrealized losses	20	0
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	5,143	245
Continuous unrealized loss position, less than 12 months, gross unrealized losses	107	2
Continuous unrealized loss position, 12 months or longer, fair value	116	0
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	4	0
Continuous unrealized loss position, total, fair value	5,259	245
Continuous unrealized loss position, total, gross unrealized losses	111	2
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	391	7
Continuous unrealized loss position, less than 12 months, gross unrealized losses	5	0
Continuous unrealized loss position, 12 months or longer, fair value	2	0
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	0
Continuous unrealized loss position, total, fair value	393	7
Continuous unrealized loss position, total, gross unrealized losses	5	0
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	233	20
Continuous unrealized loss position, less than 12 months, gross unrealized losses	1	0
Continuous unrealized loss position, 12 months or longer, fair value	1	1
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	0
Continuous unrealized loss position, total, fair value	234	21
Continuous unrealized loss position, total, gross unrealized losses	1	0
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	5,831	681
Continuous unrealized loss position, less than 12 months, gross unrealized losses	121	6
Continuous unrealized loss position, 12 months or longer, fair value	189	97
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	8	3
Continuous unrealized loss position, total, fair value	6,020	778
Continuous unrealized loss position, total, gross unrealized losses	$ 129	$ 9