Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 35	$ 33
Interest cost on benefit obligation	21	29
Expected return on plan assets	(69)	(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	27	23
Total non-service cost (benefit)	(21)	(17)
Net periodic benefit cost (benefit)	14	16
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	14	14
Total non-service cost (benefit)	(9)	(7)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	21	19
Total non-service cost (benefit)	(12)	(10)
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)	(2)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	(1)	0
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)