Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 34	$ 30	$ 100	$ 89
Interest cost on benefit obligation	29	35	86	105
Expected return on plan assets	(69)	(69)	(206)	(206)
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	0	(1)	(1)
Net periodic benefit cost, amortization of unrecognized net actuarial (gain) loss	23	14	69	42
Total non-service cost (benefit)	(18)	(20)	(52)	(60)
Net periodic benefit cost (benefit)	16	10	48	29
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	14	13	42	37
Total non-service cost (benefit)	(8)	(9)	(22)	(25)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	20	17	58	52
Total non-service cost (benefit)	(10)	(11)	(30)	(35)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Interest cost on benefit obligation	1	2	3	6
Expected return on plan assets	0	0	0	0
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	(1)	(3)	(3)
Net periodic benefit cost, amortization of unrecognized net actuarial (gain) loss	(1)	0	(2)	0
Total non-service cost (benefit)	(1)	1	(2)	3
Net periodic benefit cost (benefit)	(1)	1	(2)	3
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)	(1)	0	(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)	$ 0	$ 1	$ (1)	$ 2