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Retirement Benefits Retirement Benefits - Fair Value Measurements of Pension and Postretirement Benefit Plan Assets (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Derivative, Fair Value, Net |  | $ 1 |  | $ 6 |  |
Derivative, Notional Amount |  | 5,991 |  | 3,606 |  |
Net assets pending transactions excluded |  | 44 |  | (2) |  |
Fair Value, Measurements, Recurring [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [2],[3] | 19,130 | [1] | 21,303 |  |
Alternative Investment, Fair Value Disclosure |  | 3,433 |  | 3,534 |  |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [2],[3] | 5,463 | [1] | 6,090 |  |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [2],[3] | 4,581 | [1] | 4,913 |  |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [2],[3] | 486 | [1] | 458 |  |
Pension Plan, Defined Benefit [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 16,678 |  | 18,573 | $ 16,791 |
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 486 |  | 458 | 437 |
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 216 |  | 232 | 206 |
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Private Placement [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 268 |  | 224 | 229 | [4]
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 16,720 | [1] | 18,575 |  |
Alternative Investment, Fair Value Disclosure | [3] | 7,247 | [1] | 8,216 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Cash and Cash Equivalents [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 350 | [1] | 585 |  |
Alternative Investment, Fair Value Disclosure | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3],[5] | 5,346 | [1] | 6,644 |  |
Alternative Investment, Fair Value Disclosure |  | 1,980 | [1],[3],[5] | 3,077 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | US Treasury and Government [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 1,169 | [1] | 1,309 |  |
Alternative Investment, Fair Value Disclosure | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 59 | [1] | 64 |  |
Alternative Investment, Fair Value Disclosure | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 3,932 | [1] | 4,163 |  |
Alternative Investment, Fair Value Disclosure | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Other Debt Obligations [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3],[5] | 942 | [1] | 1,203 |  |
Alternative Investment, Fair Value Disclosure | [3],[5] | 613 | [1] | 756 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 6,102 | [1] | 6,739 |  |
Alternative Investment, Fair Value Disclosure | [3] | 613 | [1] | 756 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 1,219 | [1] | 1,034 |  |
Alternative Investment, Fair Value Disclosure | [3] | 1,219 | [1] | 1,034 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Hedge Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 1,608 | [1] | 1,770 |  |
Alternative Investment, Fair Value Disclosure | [3] | 1,608 | [1] | 1,770 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Real Estate Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 1,029 | [1] | 884 |  |
Alternative Investment, Fair Value Disclosure | [3] | 1,029 | [1] | 884 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Private Placement [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 1,066 | [1] | 919 |  |
Alternative Investment, Fair Value Disclosure | [3] | 798 | [1] | 695 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 4,710 | [1] | 5,300 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 350 | [1] | 585 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3],[5] | 3,364 | [1] | 3,565 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury and Government [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 996 | [1] | 1,150 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Agencies Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Debt Obligations [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3],[5] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 996 | [1] | 1,150 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Private Equity Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Hedge Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Real Estate Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Private Placement [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 4,277 | [1] | 4,601 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Cash and Cash Equivalents [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3],[5] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury and Government [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 173 | [1] | 159 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Government Agencies Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 59 | [1] | 64 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 3,716 | [1] | 3,931 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Debt Obligations [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3],[5] | 329 | [1] | 447 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 4,277 | [1] | 4,601 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Private Equity Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Hedge Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Real Estate Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Private Placement [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 486 | [1] | 458 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Cash and Cash Equivalents [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3],[5] | 2 | [1] | 2 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury and Government [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Government Agencies Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 216 | [1] | 232 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Debt Obligations [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3],[5] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 216 | [1] | 232 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Private Equity Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Hedge Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Real Estate Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 0 | [1] | 0 |  |
Pension Plan, Defined Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Private Placement [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 268 | [1] | 224 |  |
Other Postretirement Benefits Plan [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 2,408 |  | 2,732 | $ 2,578 |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 2,410 | [1] | 2,728 |  |
Alternative Investment, Fair Value Disclosure | [2],[3] | 8,600 | [1] | 9,842 |  |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] | Cash and Cash Equivalents [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 22 | [1] | 29 |  |
Alternative Investment, Fair Value Disclosure | [3] | 0 | [1] | 0 |  |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 1,047 | [1] | 1,289 |  |
Alternative Investment, Fair Value Disclosure | [3] | 508 | [1] | 764 |  |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] | US Treasury and Government [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 67 | [1] | 69 |  |
Alternative Investment, Fair Value Disclosure | [3] | 0 | [1] | 0 |  |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 126 | [1] | 136 |  |
Alternative Investment, Fair Value Disclosure | [3] | 0 | [1] | 0 |  |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 48 | [1] | 47 |  |
Alternative Investment, Fair Value Disclosure | [3] | 0 | [1] | 0 |  |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] | Other Debt Obligations [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 425 | [1] | 481 |  |
Alternative Investment, Fair Value Disclosure | [3] | 170 | [1] | 185 |  |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 666 | [1] | 733 |  |
Alternative Investment, Fair Value Disclosure | [3] | 170 | [1] | 185 |  |
Other Postretirement Benefits Plan [Member] | Fair Value, Measurements, Recurring [Member] | Hedge Funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | [3] | 411 | [1] | 430 |  |
Alternative Investment, Fair Value Disclosure | [3] | 411 | [1] | 430 |  |
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[1] | Effective March 31, 2017, Exelon became sponsor of FitzPatrick's defined benefit pension and other postretirement benefit plans, and assumed FitzPatrick's benefit plan obligations
[2] | Excludes net liabilities of $44 million and net assets of $2 million at December 31, 2018 and 2017, respectively, which are required to reconcile to the fair value of net plan assets. These items consist primarily of receivables or payables related to pending securities sales and purchases, interest and dividends receivable
[3] | See Note 11—Fair Value of Financial Assets and Liabilities for a description of levels within the fair value hierarchy
[4] | (a)Prior year amounts reflect a reclassification from Not subject to leveling into Level 3.
[5] | Includes derivative instruments of less than $1 million and $6 million, which have a total notional amount of $5,991 million and $3,606 million at December 31, 2018 and 2017, respectively. The notional principal amounts for these instruments provide one measure of the transaction volume outstanding as of the fiscal years ended and do not represent the amount of the company’s exposure to credit or market loss

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