Pension Plans, Retirement Benefits and Savings Plans (details) - Additional Pension Disclosures - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ (29)	$ (16)	$ (66)
Pension Plans [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	3,770	3,480
Aggregate projected benefit obligation for pension plans with an accumulated benefit obligation in excess of plan assets	200	200
Aggregate accumulated benefit obligation for pension plans with an accumulated benefit obligation in excess of plan assets	200	200
Fair value of plan assets for pension plans with an accumulated benefit obligation in excess of plan assets	100	100
Components of net periodic benefit cost
Service cost	119	118	131
Interest cost on benefit obligation	127	122	144
Expected return on plan assets	(240)	(230)	(230)
Net periodic benefit cost, settlement	3	1
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	(1)	(1)
Net periodic benefit cost, amortization of unrecognized net actuarial loss (gain)	85	66	96
Total non-service cost (benefit)	(26)	(42)	9
Net periodic benefit cost	93	76	140
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, net actuarial loss (gain)	40	66	43
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, foreign currency exchange rate change	2	(2)
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, settlement	(2)	(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)	(85)	(66)	(96)
Total other changes in benefit plan assets and benefit obligations recognized in other comprehensive income	(44)	(2)	(52)
Total other changes recognized in net periodic benefit cost and other comprehensive income	49	74	88
Estimated net actuarial gain (loss) that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next fiscal year	(91)
Estimated prior service benefit to be amortized over the next fiscal year	1
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Components of net periodic benefit cost
Service cost	48	48	53
Total non-service cost (benefit)	(11)	(18)	4
Pension Plans [Member] | General and administrative expenses [Member]
Components of net periodic benefit cost
Service cost	71	70	78
Total non-service cost (benefit)	(15)	(24)	5
Pension Plans [Member] | United States [Member] | Qualified Plan [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	3,550	3,260
Required contributions made	0	0	0
Required contributions during next fiscal year	0
Components of net periodic benefit cost
Service cost	112	111
Interest cost on benefit obligation	120	114
Nonqualified and Foreign Pension Plans [Member]
Defined benefit plan disclosure
Total accumulated benefit obligation	220	220
Components of net periodic benefit cost
Service cost	7	7
Interest cost on benefit obligation	7	8
Postretirement Benefit Plans [Member]
Components of net periodic benefit cost
Interest cost on benefit obligation	7	8	10
Net periodic benefit cost, amortization of unrecognized prior service benefit	(4)	(3)	(3)
Net periodic benefit cost, amortization of unrecognized net actuarial loss (gain)			1
Total non-service cost (benefit)	3	5	8
Net periodic benefit cost	3	5	8
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			(11)
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, net actuarial loss (gain)	13	(17)	(3)
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, foreign currency exchange rate change	(1)	1
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, amortization of prior service benefit	4	3	3
Other changes in benefit plan assets and benefit obligations recognized in other comprehensive income, amortization of net actuarial gain (loss)			(1)
Total other changes in benefit plan assets and benefit obligations recognized in other comprehensive income	16	(13)	(12)
Total other changes recognized in net periodic benefit cost and other comprehensive income	19	(8)	(4)
Estimated net actuarial gain (loss) that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next fiscal year	1
Estimated prior service benefit to be amortized over the next fiscal year	4
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Components of net periodic benefit cost
Total non-service cost (benefit)	1	2	3
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Components of net periodic benefit cost
Total non-service cost (benefit)	$ 2	$ 3	$ 5