Pension Plans, Retirement Benefits and Savings Plans (Details) - Additional Pension Disclosures - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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	$ (18)	$ (33)	$ 56
Pension Plans [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	4,400	4,050
Aggregate projected benefit obligation for plans with a projected benefit obligation in excess of plan assets	250	228
Aggregate plan assets for plans with a projected benefit obligation in excess of plan assets	109	100
Aggregate accumulated benefit obligation for plans with an accumulated benefit obligation in excess of plan assets	239	218
Aggregate plan assets for plans with an accumulated benefit obligation in excess of plan assets	109	100
Actuarial loss	356	470
Company contributions	9	8
Components of net periodic benefit cost (benefit)
Service cost	133	118	133
Interest cost on benefit obligation	114	141	126
Expected return on plan assets	(275)	(275)	(264)
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	(1)	(1)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	93	56	91
Total non-service cost (benefit)	(69)	(79)	(48)
Net periodic benefit cost (benefit)	64	39	85
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	1	0	0
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, net actuarial (gain) loss	61	49	160
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, foreign currency exchange rate change	1	1	(1)
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 gain (loss)	(93)	(56)	(91)
Total other changes in benefit plan assets and benefit obligations recognized in other comprehensive income	(29)	(5)	69
Total other changes recognized in net periodic benefit cost and other comprehensive income	35	34	154
Pension Plans [Member] | Net investment income [Member]
Components of net periodic benefit cost (benefit)
Service cost	1	1	1
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Components of net periodic benefit cost (benefit)
Service cost	55	48	54
Total non-service cost (benefit)	(29)	(33)	(19)
Pension Plans [Member] | General and administrative expenses [Member]
Components of net periodic benefit cost (benefit)
Service cost	77	69	78
Total non-service cost (benefit)	(40)	(46)	(29)
Pension Plans [Member] | United States [Member] | Qualified Plan [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	4,150	3,820
Actuarial loss	336	451
Required contributions made	0	0	0
Company contributions	0	0	200
Required contributions during next fiscal year	0
Components of net periodic benefit cost (benefit)
Service cost	128	112
Interest cost on benefit obligation	109	134
Nonqualified and Foreign Pension Plans [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	250	230
Actuarial loss	20	19
Company contributions	9	8
Components of net periodic benefit cost (benefit)
Service cost	5	6
Interest cost on benefit obligation	5	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	170	171
Aggregate plan assets for plans with an accumulated benefit obligation in excess of plan assets	11	12
Actuarial loss	5	(31)
Company contributions	8	8
Components of net periodic benefit cost (benefit)
Service cost	0	0	0
Interest cost on benefit obligation	4	7	7
Expected return on plan assets	0	(1)	0
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(3)	(3)	(4)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	(4)	0	0
Total non-service cost (benefit)	(3)	3	3
Net periodic benefit cost (benefit)	(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 (gain) loss	4	(31)	(18)
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, foreign currency exchange rate change	0	0	0
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, amortization of prior service benefit	3	3	4
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, amortization of net actuarial gain (loss)	4	0	0
Total other changes in benefit plan assets and benefit obligations recognized in other comprehensive income	11	(28)	(14)
Total other changes recognized in net periodic benefit cost and other comprehensive income	8	(25)	(11)
Postretirement Benefit Plans [Member] | Net investment income [Member]
Components of net periodic benefit cost (benefit)
Service cost	0	0	0
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Components of net periodic benefit cost (benefit)
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 (benefit)
Service cost	0	0	0
Total non-service cost (benefit)	$ (2)	$ 2	$ 2