Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 27	$ 36
Interest cost on benefit obligation	44	25
Expected return on plan assets	(78)	(74)
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	0	0
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	0	12
Total non-service cost (benefit)	(34)	(37)
Net periodic benefit cost (benefit)	(7)	(1)
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	11	15
Total non-service cost (benefit)	(14)	(15)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	16	21
Total non-service cost (benefit)	(20)	(22)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0
Interest cost on benefit obligation	1	1
Expected return on plan assets	0	0
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	(1)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	(2)	(1)
Total non-service cost (benefit)	(2)	(1)
Net periodic benefit cost (benefit)	(2)	(1)
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	0	0
Total non-service cost (benefit)	(1)	0
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	0	0
Total non-service cost (benefit)	$ (1)	$ (1)