Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 29	$ 33	$ 59	$ 66
Interest cost on benefit obligation	35	32	70	63
Expected return on plan assets	(68)	(66)	(137)	(132)
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	(1)	(1)	(1)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	14	23	28	46
Total non-service cost (benefit)	(20)	(12)	(40)	(24)
Net periodic benefit cost	9	21	19	42
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	12	14	24	27
Total non-service cost (benefit)	(8)	(5)	(16)	(10)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	17	19	35	39
Total non-service cost (benefit)	(12)	(7)	(24)	(14)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Interest cost on benefit obligation	2	2	4	4
Expected return on plan assets	0	0	0	0
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	(1)	(2)	(2)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	0	0	0	0
Total non-service cost (benefit)	1	1	2	2
Net periodic benefit cost	1	1	2	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)	1	1	1	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