Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 10,033	$ 11,060	$ 10,892	$ 11,072
Alternative investment	4,925	5,619
Pension Plan | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24	27	11
Pension Plan | Investment Grade Bonds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	109	163
Pension Plan | High Yield Bonds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	388	483
Pension Plan | High Yield Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	7	11
Pension Plan | Emerging Market Bonds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	14
Pension Plan | Emerging Market Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1	0
Pension Plan | Diversified Strategies
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	538
Pension Plan | U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	150	73
Pension Plan | Non-U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	500	627
Pension Plan | Emerging Market Stocks
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	75	98
Pension Plan | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	347	460
Pension Plan | Private Debt
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	452	374
Pension Plan | Market Neutral Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	746	769
Pension Plan | Directional Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	512	636
Pension Plan | Real estate
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	821	903
Pension Plan | Multi-Asset Strategies
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	763	424
Pension Plan | Cash Equivalents and Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	62	57
Pension Plan | Fair Value, Measurements, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,108	5,441
Pension Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,326	2,827
Pension Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,758	2,587
Pension Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24	27
Pension Plan | Fair Value, Measurements, Recurring | Investment Grade Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,851	1,747
Pension Plan | Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	458	432
Pension Plan | Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,393	1,315
Pension Plan | Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | High Yield Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	284	582
Pension Plan | Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	277	575
Pension Plan | Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	7
Pension Plan | Fair Value, Measurements, Recurring | Emerging Market Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	332	437
Pension Plan | Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151	217
Pension Plan | Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	181	219
Pension Plan | Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Pension Plan | Fair Value, Measurements, Recurring | U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	768	1,035
Pension Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	764	1,030
Pension Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	2
Pension Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	3
Pension Plan | Fair Value, Measurements, Recurring | Non-U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	601	706
Pension Plan | Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	601	706
Pension Plan | Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Private Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	15
Pension Plan | Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	15
Pension Plan | Fair Value, Measurements, Recurring | Multi-Asset Strategies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	342	440
Pension Plan | Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	342	440
Pension Plan | Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	2
Pension Plan | Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	2
Pension Plan | Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(2)	0
Pension Plan | Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Cash Equivalents and Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	910	477
Pension Plan | 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	3	0
Pension Plan | 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	907	476
Pension Plan | 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	0	1
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	23	$ 53	$ 193
Alternative investment	18	22
Other Postretirement Benefits Plan | Investment Grade Bonds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | High Yield Bonds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | Emerging Market Bonds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | Diversified Strategies
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | Non-U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | Emerging Market Stocks
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	6	10
Other Postretirement Benefits Plan | Private Debt
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	1	1
Other Postretirement Benefits Plan | Market Neutral Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | Directional Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | Real estate
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	1
Other Postretirement Benefits Plan | Multi-Asset Strategies
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	0	0
Other Postretirement Benefits Plan | Cash Equivalents and Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Alternative investment	11	10
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	0
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 0