Postretirement Benefit Plans (Summary of Plan Assets, Benefit Obligation and Funded Status) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 691	$ 605
Projected benefit obligation	(1,199)	(1,008)
Unfunded status of the plans	(508)	(403)
Amounts recognized in the balance sheet
Other non-current assets	27	58
Accrued and other current liabilities	(16)	(16)
Accrued post-retirement benefits	(519)	(445)
Net amount recognized	(508)	(403)
Accumulated other comprehensive income:
Net actuarial losses	(427)	(349)
Prior service credit	6	4
Total	(421)	(345)
Pension
Defined Benefit Plan Disclosure
Fair value of plan assets	691	605
Projected benefit obligation	(1,155)	(959)
Unfunded status of the plans	(464)	(354)
Amounts recognized in the balance sheet
Other non-current assets	27	58
Accrued and other current liabilities	(13)	(13)
Accrued post-retirement benefits	(478)	(399)
Net amount recognized	(464)	(354)
Accumulated other comprehensive income:
Net actuarial losses	(409)	(330)
Prior service credit	(3)	(3)
Total	(412)	(333)
Other Benefits
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Projected benefit obligation	(44)	(49)	$ (52)
Unfunded status of the plans	(44)	(49)
Amounts recognized in the balance sheet
Other non-current assets	0	0
Accrued and other current liabilities	(3)	(3)
Accrued post-retirement benefits	(41)	(46)
Net amount recognized	(44)	(49)
Accumulated other comprehensive income:
Net actuarial losses	(18)	(19)
Prior service credit	9	7
Total	(9)	(12)
Global stock funds/securities
Defined Benefit Plan Disclosure
Fair value of plan assets	118	103
Diversified growth and income funds
Defined Benefit Plan Disclosure
Fair value of plan assets	26	9
Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	105	91
Government bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	47	62
Hedging instruments
Defined Benefit Plan Disclosure
Fair value of plan assets	6	39
Hedge funds
Defined Benefit Plan Disclosure
Fair value of plan assets	1	12
Insurance contracts and other
Defined Benefit Plan Disclosure
Fair value of plan assets	384	16
Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	43	488
Level 1 | Global stock funds/securities
Defined Benefit Plan Disclosure
Fair value of plan assets	38	90
Level 1 | Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	1	86
Level 1 | Government bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	0	35
Level 1 | Hedging instruments
Defined Benefit Plan Disclosure
Fair value of plan assets	0	4
Level 1 | Insurance contracts and other
Defined Benefit Plan Disclosure
Fair value of plan assets
Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	188	35
Level 2 | Global stock funds/securities
Defined Benefit Plan Disclosure
Fair value of plan assets	66	0
Level 2 | Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	97	0
Level 2 | Government bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	19	0
Level 2 | Hedging instruments
Defined Benefit Plan Disclosure
Fair value of plan assets	6	35
Level 2 | Insurance contracts and other
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	384	13
Level 3 | Global stock funds/securities
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Level 3 | Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Level 3 | Government bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Level 3 | Insurance contracts and other
Defined Benefit Plan Disclosure
Fair value of plan assets	384	13
NAV Practical Expedient
Defined Benefit Plan Disclosure
Fair value of plan assets	76	69
NAV Practical Expedient | Global stock funds/securities
Defined Benefit Plan Disclosure
Fair value of plan assets	14	13
NAV Practical Expedient | Diversified growth and income funds
Defined Benefit Plan Disclosure
Fair value of plan assets	26	9
NAV Practical Expedient | Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	7	5
NAV Practical Expedient | Government bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	28	27
NAV Practical Expedient | Hedge funds
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 1	12
NAV Practical Expedient | Insurance contracts and other
Defined Benefit Plan Disclosure
Fair value of plan assets		$ 3