Employee Benefit Plans - Contributions (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount			$ 54	$ 61	$ 66
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			836
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year			192
2018			769
2019			547
2020			564
2021			553
2022			581
Years 2023-2027			2,892
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			64	$ 61
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year			62
2018			62
2019			60
2020			58
2021			57
2022			55
Years 2023-2027			$ 239
Discount rate assumptions used to determine benefit obligations			3.50%	3.90%	4.10%
Discount rate assumptions used to determine net periodic benefit costs			3.90%	4.10%	3.80%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year			6.80%	7.20%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate			4.80%	4.80%
Year that the rate reaches the ultimate trend rate			2026
Effect on total service and interest cost components - 1% increase			$ 2
Effect on total service and interest cost components - 1% decrease			(2)
Effect on post-retirement benefit obligation - 1% increase			53
Effect on post-retirement benefit obligation - 1% decrease			(46)
Other Postretirement Benefits Plan [Member] | Forecasted [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate assumptions used to determine net periodic benefit costs		3.50%
Domestic Plan [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			675	$ 24
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year			76
2018			528
2019			301
2020			312
2021			293
2022			315
Years 2023-2027			$ 1,456
Discount rate assumptions used to determine benefit obligations			3.60%	4.00%	4.30%
Rate of compensation increase assumptions used to determine benefit obligation			0.20%	0.20%	0.20%
Discount rate assumptions used to determine net periodic benefit costs			4.00%	4.30%	3.90%
Expected return on plan assets assumptions used to determine net periodic benefit cost			7.00%	7.50%	7.50%
Rate of compensation increase assumptions used to determine net periodic benefit cost			0.20%	0.20%	0.20%
Domestic Plan [Member] | Pension Plan [Member] | Forecasted [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate assumptions used to determine net periodic benefit costs		3.60%
Expected return on plan assets assumptions used to determine net periodic benefit cost		5.80%
Rate of compensation increase assumptions used to determine net periodic benefit cost		0.20%
Domestic Plan [Member] | Pension Plan [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year			$ 0
Domestic Plan [Member] | Pension Plan [Member] | Voluntary Contributions [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year			50
Domestic Plan [Member] | Pension Plan [Member] | Qualified Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			650
Domestic Plan [Member] | Pension Plan [Member] | Qualified Plan [Member] | Original Forecast [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			135
Domestic Plan [Member] | Pension Plan [Member] | Qualified Plan [Member] | Incremental to Forecast [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 500
Domestic Plan [Member] | Pension Plan [Member] | Qualified Plan [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			15
Domestic Plan [Member] | Pension Plan [Member] | Qualified Plan [Member] | Voluntary Contributions [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			635
Foreign Plan [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			161	$ 154
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year			116
2018			241
2019			246
2020			252
2021			260
2022			266
Years 2023-2027			$ 1,436
Discount rate assumptions used to determine benefit obligations			2.30%	2.50%	3.30%
Rate of compensation increase assumptions used to determine benefit obligation			2.60%	2.60%	2.70%
Discount rate assumptions used to determine net periodic benefit costs			2.50%	3.30%	3.10%
Expected return on plan assets assumptions used to determine net periodic benefit cost			4.10%	4.80%	5.20%
Rate of compensation increase assumptions used to determine net periodic benefit cost			2.60%	2.70%	2.60%
Foreign Plan [Member] | Pension Plan [Member] | Forecasted [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate assumptions used to determine net periodic benefit costs		2.30%
Expected return on plan assets assumptions used to determine net periodic benefit cost		3.80%
Rate of compensation increase assumptions used to determine net periodic benefit cost		2.60%
Foreign Plan [Member] | Pension Plan [Member] | Qualified Plan [Member] | Voluntary Contributions [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 105