Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 30	$ 33	$ 89	$ 99
Interest cost on benefit obligation	35	32	105	95
Expected return on plan assets	(69)	(66)	(206)	(198)
Net periodic benefit cost, amortization of unrecognized prior service benefit	0	0	(1)	(1)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	14	22	42	68
Total non-service cost (benefit)	(20)	(12)	(60)	(36)
Net periodic benefit cost	10	21	29	63
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	13	13	37	40
Total non-service cost (benefit)	(9)	(4)	(25)	(14)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	17	20	52	59
Total non-service cost (benefit)	(11)	(8)	(35)	(22)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Interest cost on benefit obligation	2	1	6	5
Expected return on plan assets	0	0	0	0
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	(1)	(3)	(3)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	0	0	0	0
Total non-service cost (benefit)	1	0	3	2
Net periodic benefit cost	1	0	3	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)	0	0	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)	$ 1	$ 0	$ 2	$ 1