Investments (Details) - Investment Information, Losses - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 9,480	$ 2,900
Continuous unrealized loss position, less than 12 months, gross unrealized losses	400	19
Continuous unrealized loss position, 12 months or longer, fair value	207	1,145
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	23	10
Continuous unrealized loss position, total, fair value	9,687	4,045
Continuous unrealized loss position, total, gross unrealized losses	423	29
Unrealized investment losses on fixed maturity investments for which fair value is less than 80% of amortized cost, total	$ 107
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of the combined fixed maturity and equity security portfolios on a pre-tax basis (less than)	1.00%
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of shareholders' equity on an after-tax basis (less than)	1.00%
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	$ 71	5
Continuous unrealized loss position, less than 12 months, gross unrealized losses	1	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	71	198
Continuous unrealized loss position, total, gross unrealized losses	1	0
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	703	668
Continuous unrealized loss position, less than 12 months, gross unrealized losses	12	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	703	680
Continuous unrealized loss position, total, gross unrealized losses	12	6
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	5	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	5	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	431	399
Continuous unrealized loss position, less than 12 months, gross unrealized losses	5	2
Continuous unrealized loss position, 12 months or longer, fair value	19	131
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0	1
Continuous unrealized loss position, total, fair value	450	530
Continuous unrealized loss position, total, gross unrealized losses	5	3
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	8,239	1,571
Continuous unrealized loss position, less than 12 months, gross unrealized losses	382	10
Continuous unrealized loss position, 12 months or longer, fair value	188	662
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	23	8
Continuous unrealized loss position, total, fair value	8,427	2,233
Continuous unrealized loss position, total, gross unrealized losses	405	$ 18
Unrealized investment losses on fixed maturity investments for which fair value is less than 80% of amortized cost for three months or less	107
Redeemable preferred stock [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	31
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0
Continuous unrealized loss position, 12 months or longer, fair value	0
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0
Continuous unrealized loss position, total, fair value	31
Continuous unrealized loss position, total, gross unrealized losses	$ 0