Investments (Details) - Investment Information, Losses - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 9,210	$ 1,045
Continuous unrealized loss position, less than 12 months, gross unrealized losses	292	8
Continuous unrealized loss position, 12 months or longer, fair value	126	98
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	4	3
Continuous unrealized loss position, total, fair value	9,336	1,143
Continuous unrealized loss position, total, gross unrealized losses	296	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	854	92
Continuous unrealized loss position, less than 12 months, gross unrealized losses	18	0
Continuous unrealized loss position, 12 months or longer, fair value	0	0
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	0
Continuous unrealized loss position, total, fair value	854	92
Continuous unrealized loss position, total, gross unrealized losses	18	0
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	4,654	245
Continuous unrealized loss position, less than 12 months, gross unrealized losses	132	2
Continuous unrealized loss position, 12 months or longer, fair value	0	0
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	0
Continuous unrealized loss position, total, fair value	4,654	245
Continuous unrealized loss position, total, gross unrealized losses	132	2
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	158	7
Continuous unrealized loss position, less than 12 months, gross unrealized losses	4	0
Continuous unrealized loss position, 12 months or longer, fair value	0	0
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	0
Continuous unrealized loss position, total, fair value	158	7
Continuous unrealized loss position, total, gross unrealized losses	4	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	50	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	51	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	3,494	681
Continuous unrealized loss position, less than 12 months, gross unrealized losses	137	6
Continuous unrealized loss position, 12 months or longer, fair value	125	97
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	4	3
Continuous unrealized loss position, total, fair value	3,619	778
Continuous unrealized loss position, total, gross unrealized losses	$ 141	$ 9