Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 33	$ 30
Interest cost on benefit obligation	31	31
Expected return on plan assets	(66)	(60)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	23	18
Total non-service cost (benefit)	(12)	(11)
Net periodic benefit cost	21	19
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	13	12
Total non-service cost (benefit)	(5)	(4)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	20	18
Total non-service cost (benefit)	(7)	(7)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Interest cost on benefit obligation	2	2
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	(1)
Total non-service cost (benefit)	1	1
Net periodic benefit cost	1	1
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Total non-service cost (benefit)	$ 1	$ 1