Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 33	$ 30	$ 66	$ 60
Interest cost on benefit obligation	32	30	63	61
Expected return on plan assets	(66)	(60)	(132)	(120)
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)		(1)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	23	19	46	37
Total non-service cost (benefit)	(12)	(11)	(24)	(22)
Net periodic benefit cost	21	19	42	38
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	14	12	27	24
Total non-service cost (benefit)	(5)	(5)	(10)	(9)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	19	18	39	36
Total non-service cost (benefit)	(7)	(6)	(14)	(13)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Interest cost on benefit obligation	2	1	4	3
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	$ (1)	(2)	(2)
Total non-service cost (benefit)	1		2	1
Net periodic benefit cost	1		2	1
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Total non-service cost (benefit)	$ 1		1
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Total non-service cost (benefit)			$ 1	$ 1