Post-Employment Benefits - Defined Benefit Pension Plan Assets (Details) - Defined benefit plans - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		$ 9,702	$ 7,116	$ 5,637
Fair value measured at Levels 1, 2 and 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		4,230	2,851
Fair value measured at Levels 1, 2 and 3 | U.S. large cap
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[1]	1,143	884
Fair value measured at Levels 1, 2 and 3 | U.S. mid cap
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[2]	164	138
Fair value measured at Levels 1, 2 and 3 | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[3]	524	349
Fair value measured at Levels 1, 2 and 3 | U.S. government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	132	149
Fair value measured at Levels 1, 2 and 3 | Corporate debt instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	854	372
Fair value measured at Levels 1, 2 and 3 | Non-U.S. government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	544	202
Fair value measured at Levels 1, 2 and 3 | Other fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	297	320
Fair value measured at Levels 1, 2 and 3 | Absolute return funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[5]	310	296
Fair value measured at Levels 1, 2 and 3 | Real assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		10	9
Fair value measured at Levels 1, 2 and 3 | Other plan assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[6]	252	132
Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		2,982	2,051
Quoted prices in active markets for identical assets (Level 1) | U.S. large cap
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[1]	1,143	884
Quoted prices in active markets for identical assets (Level 1) | U.S. mid cap
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[2]	164	138
Quoted prices in active markets for identical assets (Level 1) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[3]	524	349
Quoted prices in active markets for identical assets (Level 1) | U.S. government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	18	21
Quoted prices in active markets for identical assets (Level 1) | Corporate debt instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	178	112
Quoted prices in active markets for identical assets (Level 1) | Non-U.S. government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	397	84
Quoted prices in active markets for identical assets (Level 1) | Other fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	294	318
Quoted prices in active markets for identical assets (Level 1) | Absolute return funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[5]	4	4
Quoted prices in active markets for identical assets (Level 1) | Real assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		10	9
Quoted prices in active markets for identical assets (Level 1) | Other plan assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[6]	250	132
Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		1,248	800
Significant other observable inputs (Level 2) | U.S. large cap
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[1]	0	0
Significant other observable inputs (Level 2) | U.S. mid cap
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[2]	0	0
Significant other observable inputs (Level 2) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[3]	0	0
Significant other observable inputs (Level 2) | U.S. government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	114	128
Significant other observable inputs (Level 2) | Corporate debt instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	676	260
Significant other observable inputs (Level 2) | Non-U.S. government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	147	118
Significant other observable inputs (Level 2) | Other fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	3	2
Significant other observable inputs (Level 2) | Absolute return funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[5]	306	292
Significant other observable inputs (Level 2) | Real assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		0	0
Significant other observable inputs (Level 2) | Other plan assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[6]	2	0
Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		0	0
Significant unobservable inputs (Level 3) | U.S. large cap
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[1]	0	0
Significant unobservable inputs (Level 3) | U.S. mid cap
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[2]	0	0
Significant unobservable inputs (Level 3) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[3]	0	0
Significant unobservable inputs (Level 3) | U.S. government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	0	0
Significant unobservable inputs (Level 3) | Corporate debt instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	0	0
Significant unobservable inputs (Level 3) | Non-U.S. government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	0	0
Significant unobservable inputs (Level 3) | Other fixed income securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[4]	0	0
Significant unobservable inputs (Level 3) | Absolute return funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[5]	0	0
Significant unobservable inputs (Level 3) | Real assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		0	0
Significant unobservable inputs (Level 3) | Other plan assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	[6]	0	0
Total assets measured at NAV
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets		$ 5,472	$ 4,265

[1] A mix of index funds and actively managed equity accounts that are benchmarked to various large cap indices.

[2] A mix of index funds and actively managed equity accounts that are benchmarked to various mid cap indices.

[3] A mix of index funds and actively managed equity accounts that are benchmarked to various non-U.S. equity indices in both developed and emerging markets.

[4] Securities held by actively managed accounts, index funds and mutual funds.

[5] Primarily funds having global mandates with the flexibility to allocate capital broadly across a wide range of asset classes and strategies, including but not limited to equities, fixed income, commodities, financial futures, currencies and other securities, with objectives to outperform agreed upon benchmarks of specific return and volatility targets.

[6] Investments in cash and cash equivalents.