Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 30	$ 33
Interest cost on benefit obligation	35	31
Expected return on plan assets	(69)	(66)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	14	23
Total non-service cost (benefit)	(20)	(12)
Net periodic benefit cost	10	21
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	12	13
Total non-service cost (benefit)	(8)	(5)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	18	20
Total non-service cost (benefit)	(12)	(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