Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 36	$ 35
Interest cost on benefit obligation	25	21
Expected return on plan assets	(74)	(69)
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	12	27
Total non-service cost (benefit)	(37)	(21)
Net periodic benefit cost (benefit)	(1)	14
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	15	14
Total non-service cost (benefit)	(15)	(9)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	21	21
Total non-service cost (benefit)	(22)	(12)
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	(1)	(1)
Total non-service cost (benefit)	(1)	(1)
Net periodic benefit cost (benefit)	(1)	(1)
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	0	0
Total non-service cost (benefit)	0	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)