Employee Benefits - Fair Value of Plan Assets and Liabilities (Details) - Combined Pension Plan - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	$ 10,546	$ 10,493	$ 10,033	$ 11,060
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	8	22	24
Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	3,368	3,864
Investment Grade Bonds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	352	211
High Yield Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	6	5	7
High Yield Bonds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	25	39
U.S. Stocks | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	2	1	2
U.S. Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	192	169
Non-U.S. Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	308	467
Emerging Market Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	81	92
Private Equity | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	283	322
Private Debt | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	16	$ 15
Private Debt | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	505	483
Market Neutral Hedge Funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	222	433
Directional Hedge Funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	254	443
Real Estate | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	543	635
Multi-Asset Strategies | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	375	449
Cash equivalents and short-term investments | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	228	121
Fair Value, Measurements, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	10,546	10,493
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	3,368	3,864
Fair Value, Measurements, Recurring | Defined Benefit Plan, Assets Before Liabilities | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	2,389	2,636
Fair Value, Measurements, Recurring | Defined Benefit Plan, Assets Before Liabilities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	4,782	3,988
Fair Value, Measurements, Recurring | Defined Benefit Plan, Assets Before Liabilities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	8	22
Fair Value, Measurements, Recurring | Defined Benefit Plan, Assets Before Liabilities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	7,179	6,646
Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	726	828
Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	4,066	3,197
Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	4,792	4,025
Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	262	232
Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	6	5
Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	268	237
Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	218	203
Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	172	84
Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	390	287
Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	653	756
Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	3
Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	2	1
Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	655	760
Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	593	592
Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	1	0
Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	594	592
Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	16
Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	16
Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	199	257
Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	199	257
Fair Value, Measurements, Recurring | Repurchase Agreements | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Repurchase Agreements | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	39
Fair Value, Measurements, Recurring | Repurchase Agreements | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Repurchase Agreements | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	39
Fair Value, Measurements, Recurring | Cash equivalents and short-term investments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Cash equivalents and short-term investments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	281	433
Fair Value, Measurements, Recurring | Cash equivalents and short-term investments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Cash equivalents and short-term investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	281	433
Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	1
Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	(1)	(18)
Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	0	0
Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets (liabilities)	$ (1)	$ (17)