Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 33	$ 29	$ 66	$ 59
Interest cost on benefit obligation	28	35	57	70
Expected return on plan assets	(68)	(68)	(137)	(137)
Net periodic benefit cost, amortization of unrecognized prior service benefit	0	(1)	0	(1)
Net periodic benefit cost, amortization of unrecognized net actuarial (gain) loss	23	14	46	28
Total non-service cost (benefit)	(17)	(20)	(34)	(40)
Net periodic benefit cost (benefit)	16	9	32	19
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	14	12	28	24
Total non-service cost (benefit)	(7)	(8)	(14)	(16)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	19	17	38	35
Total non-service cost (benefit)	(10)	(12)	(20)	(24)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Interest cost on benefit obligation	1	2	2	4
Expected return on plan assets	0	0	0	0
Net periodic benefit cost, amortization of unrecognized prior service benefit	0	(1)	(2)	(2)
Net periodic benefit cost, amortization of unrecognized net actuarial (gain) loss	(1)	0	(1)	0
Total non-service cost (benefit)	0	1	(1)	2
Net periodic benefit cost (benefit)	0	1	(1)	2
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)	0	1	0	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	$ 0	$ (1)	$ 1