Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 33	$ 30
Interest cost on benefit obligation	29	35
Expected return on plan assets	(69)	(69)
Net periodic benefit cost, amortization of unrecognized prior service benefit	0	0
Net periodic benefit cost, amortization of unrecognized net actuarial loss	23	14
Total non-service cost (benefit)	(17)	(20)
Net periodic benefit cost	16	10
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	14	12
Total non-service cost (benefit)	(7)	(8)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	19	18
Total non-service cost (benefit)	(10)	(12)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0
Interest cost on benefit obligation	1	2
Expected return on plan assets	0	0
Net periodic benefit cost, amortization of unrecognized prior service benefit	(2)	(1)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	0	0
Total non-service cost (benefit)	(1)	1
Net periodic benefit cost	(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