Fair Value Measurements - Schedule of Pension Plan Assets at Fair Value by Level (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	$ 3,000	$ 2,825
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	3,000	2,825	$ 2,431
Pension Plans | Total
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	2,020	1,952
Pension Plans | Total | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	337	823
Pension Plans | Total | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	21	78
Pension Plans | Total | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	387	229
Pension Plans | Total | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	1,026	588
Pension Plans | Total | Government securities
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	164	217
Pension Plans | Total | Asset-backed securities, derivatives and other
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	85	17
Pension Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	745	1,130
Pension Plans | Level 1 | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	337	823
Pension Plans | Level 1 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	21	78
Pension Plans | Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	387	229
Pension Plans | Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	0	0
Pension Plans | Level 1 | Government securities
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	0	0
Pension Plans | Level 1 | Asset-backed securities, derivatives and other
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	0	0
Pension Plans | Level 2
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	1,275	822
Pension Plans | Level 2 | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	0	0
Pension Plans | Level 2 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	0	0
Pension Plans | Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	0	0
Pension Plans | Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	1,026	588
Pension Plans | Level 2 | Government securities
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	164	217
Pension Plans | Level 2 | Asset-backed securities, derivatives and other
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	85	17
Pension Plans | Investments measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Investments at fair value	$ 980	$ 873