Pension Plans and Other Postretirement Benefits - Fair Value of Pension Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 6,845	$ 6,395	$ 5,663
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,291	2,121
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,231	3,196
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	343	354	430
Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,865	5,671
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	118	92
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81	65
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	199	157
Debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	575	592
Debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,887	2,844
Debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Debt securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,462	3,437
U.s. government securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	575	592
U.s. government securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36	31
U.s. government securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.s. government securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	611	623
States, municipalities and political subdivisions | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
States, municipalities and political subdivisions | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	170	157
States, municipalities and political subdivisions | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
States, municipalities and political subdivisions | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	170	157
U.S. corporate securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. corporate securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,006	1,849
U.S. corporate securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1	5
U.S. corporate securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,006	1,850
Foreign securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	167	178
Foreign securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	167	178
Residential mortgage-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Residential mortgage-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	287	385
Residential mortgage-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Residential mortgage-backed securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	287	385
Commercial mortgage-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Commercial mortgage-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	83	89
Commercial mortgage-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Commercial mortgage-backed securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	83	89
Other asset-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other asset-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	133	150
Other asset-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other asset-backed securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	133	150
Redeemable preferred securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Redeemable preferred securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Redeemable preferred securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Redeemable preferred securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,598	1,437
Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Equity securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,598	1,438
U.S. Domestic | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,046	931
U.S. Domestic | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
U.S. Domestic | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Domestic | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,046	932
International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	537	481
International | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	537	481
Domestic real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	25
Domestic real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Domestic real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Domestic real estate | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	25
Other investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	263	286
Other investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	343	353
Other investments | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	606	639
Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	343	353	$ 425
Real estate | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	343	353
Common/collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Common/collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	266	288
Common/collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Common/collective trusts | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	266	288
Derivatives | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Derivatives | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(3)	(2)
Derivatives | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Derivatives | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(3)	(2)
Common/collective trusts, Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets, investment within plan asset category, amount		137
Common/collective trusts, Equity Securities | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Plan assets, investment within plan asset category, amount	84
Common/collective trusts, Debt Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets, investment within plan asset category, amount		151
Common/collective trusts, Debt Securities | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Plan assets, investment within plan asset category, amount	182
Private equity investments | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	624	540
Hedge fund investments | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	214	$ 184
Other receivables | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 142