Benefit Plans (Fair Values of Pension and Postretirement Plan Assets) (Details) - USD ($) | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 |
Pension [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ 8,911,000,000 | $ 8,167,000,000 | $ 7,138,000,000 |
Pension [Member] | US Treasury and Government [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 950,000,000 | 811,000,000 | |
Pension [Member] | Municipal Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 64,000,000 | 57,000,000 | |
Pension [Member] | Debt Security, Government, Non-US [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 90,000,000 | 98,000,000 | |
Pension [Member] | Corporate Debt Security, Above Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 3,760,000,000 | 3,523,000,000 | |
Pension [Member] | Corporate Debt Security, Below Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 868,000,000 | 521,000,000 | |
Pension [Member] | Common stock: International equities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 316,000,000 | 296,000,000 | |
Pension [Member] | Common stock: U.S. equities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 970,000,000 | 1,263,000,000 | |
Pension [Member] | Other, Net [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 377,000,000 | 475,000,000 | |
Pension [Member] | U S Large Cap [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ 924,000,000 | $ 825,000,000 | |
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | |
Pension [Member] | U S Small Cap [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ 455,000,000 | $ 386,000,000 | |
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | |
Pension [Member] | International Developed Markets [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ 114,000,000 | $ 106,000,000 | |
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | |
Pension [Member] | Other Investments [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ 23,000,000 | $ (194,000,000) | |
Pension [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 7,395,000,000 | 7,044,000,000 | |
Pension [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 1,307,000,000 | 1,555,000,000 | |
Pension [Member] | Level 1 [Member] | US Treasury and Government [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Pension [Member] | Level 1 [Member] | Municipal Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Pension [Member] | Level 1 [Member] | Debt Security, Government, Non-US [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Pension [Member] | Level 1 [Member] | Corporate Debt Security, Above Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Pension [Member] | Level 1 [Member] | Corporate Debt Security, Below Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Pension [Member] | Level 1 [Member] | Common stock: International equities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 316,000,000 | 296,000,000 | |
Pension [Member] | Level 1 [Member] | Common stock: U.S. equities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 970,000,000 | 1,263,000,000 | |
Pension [Member] | Level 1 [Member] | Other, Net [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 21,000,000 | (4,000,000) | |
Pension [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 6,088,000,000 | 5,489,000,000 | |
Pension [Member] | Level 2 [Member] | US Treasury and Government [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 950,000,000 | 811,000,000 | |
Pension [Member] | Level 2 [Member] | Municipal Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 64,000,000 | 57,000,000 | |
Pension [Member] | Level 2 [Member] | Debt Security, Government, Non-US [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 90,000,000 | 98,000,000 | |
Pension [Member] | Level 2 [Member] | Corporate Debt Security, Above Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 3,760,000,000 | 3,523,000,000 | |
Pension [Member] | Level 2 [Member] | Corporate Debt Security, Below Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 868,000,000 | 521,000,000 | |
Pension [Member] | Level 2 [Member] | Common stock: International equities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Pension [Member] | Level 2 [Member] | Common stock: U.S. equities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Pension [Member] | Level 2 [Member] | Other, Net [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 356,000,000 | 479,000,000 | |
Pension [Member] | NAV [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 1,493,000,000 | 1,317,000,000 | |
Postretirement Benefit Plan [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 201,000,000 | 213,000,000 | $ 211,000,000 |
Postretirement Benefit Plan [Member] | US Treasury and Government [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 6,000,000 | 11,000,000 | |
Postretirement Benefit Plan [Member] | Debt Security, Government, Non-US [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 4,000,000 | 5,000,000 | |
Postretirement Benefit Plan [Member] | Corporate Debt Security, Above Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 55,000,000 | 63,000,000 | |
Postretirement Benefit Plan [Member] | Corporate Debt Security, Below Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 11,000,000 | 9,000,000 | |
Postretirement Benefit Plan [Member] | Other, Net [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 7,000,000 | 7,000,000 | |
Postretirement Benefit Plan [Member] | U S Large Cap [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ 97,000,000 | $ 97,000,000 | |
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | |
Postretirement Benefit Plan [Member] | International Developed Markets [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ 25,000,000 | $ 24,000,000 | |
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | us_gaap:FairValueMeasuredAtNetAssetValuePerShareMember | |
Postretirement Benefit Plan [Member] | Other Investments [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ (4,000,000) | $ (3,000,000) | |
Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 83,000,000 | 95,000,000 | |
Postretirement Benefit Plan [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Postretirement Benefit Plan [Member] | Level 1 [Member] | US Treasury and Government [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Postretirement Benefit Plan [Member] | Level 1 [Member] | Debt Security, Government, Non-US [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Postretirement Benefit Plan [Member] | Level 1 [Member] | Corporate Debt Security, Above Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Postretirement Benefit Plan [Member] | Level 1 [Member] | Corporate Debt Security, Below Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Postretirement Benefit Plan [Member] | Level 1 [Member] | Other, Net [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Postretirement Benefit Plan [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 83,000,000 | 95,000,000 | |
Postretirement Benefit Plan [Member] | Level 2 [Member] | US Treasury and Government [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 6,000,000 | 11,000,000 | |
Postretirement Benefit Plan [Member] | Level 2 [Member] | Debt Security, Government, Non-US [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 4,000,000 | 5,000,000 | |
Postretirement Benefit Plan [Member] | Level 2 [Member] | Corporate Debt Security, Above Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 55,000,000 | 63,000,000 | |
Postretirement Benefit Plan [Member] | Level 2 [Member] | Corporate Debt Security, Below Investment Grade [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 11,000,000 | 9,000,000 | |
Postretirement Benefit Plan [Member] | Level 2 [Member] | Other, Net [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 7,000,000 | 7,000,000 | |
Postretirement Benefit Plan [Member] | Level 3 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | 0 | 0 | |
Postretirement Benefit Plan [Member] | NAV [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Fair value of plan assets, net | $ 122,000,000 | $ 121,000,000 | |