Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 35	$ 34	$ 106	$ 100
Interest cost on benefit obligation	21	29	62	86
Expected return on plan assets	(69)	(69)	(206)	(206)
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	(1)	(1)	(1)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	28	23	82	69
Total non-service cost (benefit)	(21)	(18)	(63)	(52)
Net periodic benefit cost (benefit)	14	16	43	48
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	14	14	43	42
Total non-service cost (benefit)	(9)	(8)	(26)	(22)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	21	20	63	58
Total non-service cost (benefit)	(12)	(10)	(37)	(30)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Interest cost on benefit obligation	1	1	2	3
Expected return on plan assets	0	0	0	0
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	(1)	(3)	(3)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	(1)	(1)	(2)	(2)
Total non-service cost (benefit)	(1)	(1)	(3)	(2)
Net periodic benefit cost (benefit)	(1)	(1)	(3)	(2)
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Total non-service cost (benefit)	0	(1)	(1)	(1)
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Total non-service cost (benefit)	$ (1)	$ 0	$ (2)	$ (1)