Pension Plans, Retirement Benefits and Savings Plans (Details) - Additional Pension Disclosures - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income:
Total other changes in benefit plan assets and benefit obligations recognized in other comprehensive income	$ (33)	$ 56	$ (29)
Pension Plans [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	4,050	3,530
Aggregate projected benefit obligation for plans with a projected benefit obligation in excess of plan assets	228	203
Aggregate plan assets for plans with a projected benefit obligation in excess of plan assets	100	87
Aggregate accumulated benefit obligation for plans with an accumulated benefit obligation in excess of plan assets	218	195
Aggregate plan assets for plans with an accumulated benefit obligation in excess of plan assets	100	87
Actuarial gain (loss)	(470)	284
Company contributions	8	210
Components of net periodic benefit cost
Service cost	118	133	119
Interest cost on benefit obligation	141	126	127
Expected return on plan assets	(275)	(264)	(240)
Net periodic benefit cost, settlement	0	0	3
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	(1)	(1)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	56	91	85
Total non-service cost (benefit)	(79)	(48)	(26)
Net periodic benefit cost	39	85	93
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income:
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, prior service benefit	0	0	0
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, net actuarial loss (gain)	49	160	40
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, foreign currency exchange rate change	1	(1)	2
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, settlement	0	0	(2)
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, amortization of prior service benefit	1	1	1
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, amortization of net actuarial loss	(56)	(91)	(85)
Total other changes in benefit plan assets and benefit obligations recognized in other comprehensive income	(5)	69	(44)
Total other changes recognized in net periodic benefit cost and other comprehensive income	34	154	49
Pension Plans [Member] | Net investment income [Member]
Components of net periodic benefit cost
Service cost	1	1	0
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Components of net periodic benefit cost
Service cost	48	54	48
Total non-service cost (benefit)	(33)	(19)	(11)
Pension Plans [Member] | General and administrative expenses [Member]
Components of net periodic benefit cost
Service cost	69	78	71
Total non-service cost (benefit)	(46)	(29)	(15)
Pension Plans [Member] | United States [Member] | Qualified Plan [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	3,820	3,320
Actuarial gain (loss)	(451)	273
Required contributions made	0	0	0
Company contributions	0	200	300
Required contributions during next fiscal year	0
Components of net periodic benefit cost
Service cost	112	126
Interest cost on benefit obligation	134	119
Nonqualified and Foreign Pension Plans [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	230	210
Actuarial gain (loss)	(19)	11
Company contributions	8	10
Components of net periodic benefit cost
Service cost	6	7
Interest cost on benefit obligation	7	7
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Aggregate accumulated benefit obligation for plans with an accumulated benefit obligation in excess of plan assets	171	203
Aggregate plan assets for plans with an accumulated benefit obligation in excess of plan assets	12	12
Actuarial gain (loss)	31	18
Company contributions	8	9
Components of net periodic benefit cost
Service cost	0	0	0
Interest cost on benefit obligation	7	7	7
Expected return on plan assets	(1)	0	0
Net periodic benefit cost, settlement	0	0	0
Net periodic benefit cost, amortization of unrecognized prior service benefit	(3)	(4)	(4)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	0	0	0
Total non-service cost (benefit)	3	3	3
Net periodic benefit cost	3	3	3
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income:
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, prior service benefit	0	0	0
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, net actuarial loss (gain)	(31)	(18)	13
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, foreign currency exchange rate change	0	0	(1)
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, settlement	0	0	0
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, amortization of prior service benefit	3	4	4
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, amortization of net actuarial loss	0	0	0
Total other changes in benefit plan assets and benefit obligations recognized in other comprehensive income	(28)	(14)	16
Total other changes recognized in net periodic benefit cost and other comprehensive income	(25)	(11)	19
Postretirement Benefit Plans [Member] | Net investment income [Member]
Components of net periodic benefit cost
Service cost	0	0	0
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Components of net periodic benefit cost
Service cost	0	0	0
Total non-service cost (benefit)	1	1	1
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Components of net periodic benefit cost
Service cost	0	0	0
Total non-service cost (benefit)	$ 2	$ 2	$ 2