Pension and Postretirement Benefit Plans and Defined Contribution Plans - Plan Assets (Details) - USD ($) $ in Millions		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 588		$ 519		$ 469
US [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		16,094		14,586
US [Member] | Pension Plan [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3,306		3,496
US [Member] | Pension Plan [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		10,103		8,451
US [Member] | Pension Plan [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1		1
US [Member] | Pension Plan [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	2,684		2,638
US [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		781		363
US [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		70		80
US [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		711		284
US [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
US [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	0		0
US [Member] | Pension Plan [Member] | Global Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3,241		3,464
US [Member] | Pension Plan [Member] | Global Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3,213		3,406
US [Member] | Pension Plan [Member] | Global Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		27		57
US [Member] | Pension Plan [Member] | Global Equity Securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1		0
US [Member] | Pension Plan [Member] | Global Equity Securities [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	0		0
US [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,325		1,179
US [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
US [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,110		819
US [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
US [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	215		360
US [Member] | Pension Plan [Member] | Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		6,499		5,292
US [Member] | Pension Plan [Member] | Corporate debt [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		23		10
US [Member] | Pension Plan [Member] | Corporate debt [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		6,476		5,281
US [Member] | Pension Plan [Member] | Corporate debt [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		1
US [Member] | Pension Plan [Member] | Corporate debt [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	0
US [Member] | Pension Plan [Member] | Government and Agency Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	1,555		1,799
US [Member] | Pension Plan [Member] | Government and Agency Obligations [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	0		0
US [Member] | Pension Plan [Member] | Government and Agency Obligations [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	1,555		1,799
US [Member] | Pension Plan [Member] | Government and Agency Obligations [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	0		0
US [Member] | Pension Plan [Member] | Government and Agency Obligations [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1],[2]	0		0
US [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		23		6
US [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets				0
US [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		23		6
US [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
US [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	0		0
US [Member] | Pension Plan [Member] | Partnership Interest [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	1,431		1,212
US [Member] | Pension Plan [Member] | Partnership Interest [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	0		0
US [Member] | Pension Plan [Member] | Partnership Interest [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]			0
US [Member] | Pension Plan [Member] | Partnership Interest [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	0		0
US [Member] | Pension Plan [Member] | Partnership Interest [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1],[3]	1,431		1,212
US [Member] | Pension Plan [Member] | Insurance Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		190		196
US [Member] | Pension Plan [Member] | Insurance Contracts [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
US [Member] | Pension Plan [Member] | Insurance Contracts [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		190		196
US [Member] | Pension Plan [Member] | Insurance Contracts [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
US [Member] | Pension Plan [Member] | Insurance Contracts [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	0		0
US [Member] | Pension Plan [Member] | Other Commingled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	1,049		1,075
US [Member] | Pension Plan [Member] | Other Commingled Funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	0		0
US [Member] | Pension Plan [Member] | Other Commingled Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	11		9
US [Member] | Pension Plan [Member] | Other Commingled Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	0		0
US [Member] | Pension Plan [Member] | Other Commingled Funds [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1],[4]	1,038		1,066
US [Member] | Postretirement Benefits Plan [Member] | Insurance Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[5]	588		519
US [Member] | Postretirement Benefits Plan [Member] | Insurance Contracts [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[5]	0
US [Member] | Postretirement Benefits Plan [Member] | Insurance Contracts [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[5]	588		519
US [Member] | Postretirement Benefits Plan [Member] | Insurance Contracts [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[5]			0
US [Member] | Postretirement Benefits Plan [Member] | Insurance Contracts [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1],[5]			0
International [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		9,811		8,956		8,215
International [Member] | Pension Plan [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		61		33
International [Member] | Pension Plan [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		5,681		4,929
International [Member] | Pension Plan [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,362		1,342
International [Member] | Pension Plan [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	2,707		2,652
International [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		407		221
International [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		61		33
International [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		346		187
International [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
International [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	0		0
International [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,051		1,922
International [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
International [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,681		1,548
International [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
International [Member] | Pension Plan [Member] | Commingled Funds Equity Securities [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	370		374
International [Member] | Pension Plan [Member] | Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		925		796
International [Member] | Pension Plan [Member] | Corporate debt [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
International [Member] | Pension Plan [Member] | Corporate debt [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		925		796
International [Member] | Pension Plan [Member] | Corporate debt [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
International [Member] | Pension Plan [Member] | Corporate debt [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	0		0
International [Member] | Pension Plan [Member] | Government and Agency Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	1,334		1,200
International [Member] | Pension Plan [Member] | Government and Agency Obligations [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	0		0
International [Member] | Pension Plan [Member] | Government and Agency Obligations [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	1,334		1,200
International [Member] | Pension Plan [Member] | Government and Agency Obligations [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	0		0
International [Member] | Pension Plan [Member] | Government and Agency Obligations [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1],[2]	0		0
International [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,484		2,201
International [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
International [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,217		1,031
International [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
International [Member] | Pension Plan [Member] | Fixed Income Commingled Funds [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	1,267		1,171
International [Member] | Pension Plan [Member] | Partnership Interest [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	69		66
International [Member] | Pension Plan [Member] | Partnership Interest [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	0		0
International [Member] | Pension Plan [Member] | Partnership Interest [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	3		3
International [Member] | Pension Plan [Member] | Partnership Interest [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	0		0
International [Member] | Pension Plan [Member] | Partnership Interest [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1],[3]	66		63
International [Member] | Pension Plan [Member] | Insurance Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,027		1,027
International [Member] | Pension Plan [Member] | Insurance Contracts [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
International [Member] | Pension Plan [Member] | Insurance Contracts [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		57		82
International [Member] | Pension Plan [Member] | Insurance Contracts [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		969		944		684
International [Member] | Pension Plan [Member] | Insurance Contracts [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	1		1
International [Member] | Pension Plan [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	1,514		1,524
International [Member] | Pension Plan [Member] | Other [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	0		0
International [Member] | Pension Plan [Member] | Other [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	117		82
International [Member] | Pension Plan [Member] | Other [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		393	[4]	398	[4]	$ 382
International [Member] | Pension Plan [Member] | Other [Member] | Assets Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1],[4]	$ 1,003		$ 1,043

[1] Certain investments that are measured at NAV per share (or its equivalent) have not been classified in the fair value hierarchy. The NAV amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented for the total pension benefits plan assets.

[2] Government and agency obligations are inclusive of repurchase agreements

[3] Mainly includes investments in private equity, private debt, public equity limited partnerships, and, to a lesser extent, real estate and venture capital.

[4] Mostly includes investments in hedge funds and real estate.

[5] Reflects postretirement plan assets, which support a portion of our U.S. retiree medical plans.