Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 33	$ 30	$ 99	$ 90
Interest cost on benefit obligation	32	35	95	96
Expected return on plan assets	(66)	(60)	(198)	(180)
Net periodic benefit cost, amortization of unrecognized prior service benefit		(1)	(1)	(1)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	22	29	68	66
Total non-service cost (benefit)	(12)	3	(36)	(19)
Net periodic benefit cost	21	33	63	71
Pension Plans [Member] | United States [Member] | Qualified Plan [Member]
Defined benefit plan disclosure
Company contributions	200
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	13	13	40	37
Total non-service cost (benefit)	(4)	1	(14)	(8)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	20	17	59	53
Total non-service cost (benefit)	(8)	2	(22)	(11)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Interest cost on benefit obligation	1	2	5	5
Net periodic benefit cost, amortization of unrecognized prior service benefit	$ (1)	(1)	(3)	(3)
Total non-service cost (benefit)		1	2	2
Net periodic benefit cost		1	2	2
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Total non-service cost (benefit)		$ 1	1	1
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Total non-service cost (benefit)			$ 1	$ 1