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Retirement Benefits (Components of the Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits expense | $ (686) | $ (535) | $ (701) | $ (749) | $ (580) | $ (517) | $ (661) | $ (673) |  | $ (2,655) | $ (2,431) | $ (2,184)
Other net benefit credits |  |  |  |  |  |  |  |  |  | (201) | (233) | (235)
Pension Plan |  |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits expense |  |  |  |  |  |  |  |  |  | 76 | 178 | 196
Other net benefit credits |  |  |  |  |  |  |  |  |  | (206) | (240) | (114)
Service cost |  |  |  |  |  |  |  |  |  | 76 | 178 | 196
Interest cost |  |  |  |  |  |  |  |  |  | 497 | 537 | 587
Expected return on plan assets |  |  |  |  |  |  |  |  |  | (921) | (940) | (977)
Amortization of prior service (credit) cost |  |  |  |  |  |  |  |  |  | (2) | (1) | (1)
Recognized actuarial loss (gain) |  |  |  |  |  |  |  |  |  | 167 | 168 | 271
Net periodic benefit (credit) cost |  |  |  |  |  |  |  |  |  | (183) | (58) | 76
Curtailment (gain) loss |  |  |  |  |  |  |  |  |  | (1) | (4) | 5
Plan termination |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Settlement loss |  |  |  |  |  |  |  |  |  | 54 | 0 | 1
Total (credit) cost |  |  |  |  |  |  |  |  |  | (130) | (62) | 82
Postretirement Benefits |  |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits expense |  |  |  |  |  |  |  |  |  | 1 | 2 | 3
Other net benefit credits |  |  |  |  |  |  |  |  |  | 5 | 7 | (121)
Service cost |  |  |  |  |  |  |  |  |  | 1 | 2 | 3
Interest cost |  |  |  |  |  |  |  |  |  | 4 | 5 | 5
Expected return on plan assets |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization of prior service (credit) cost |  |  |  |  |  |  |  |  |  | 1 | 4 | 3
Recognized actuarial loss (gain) |  |  |  |  |  |  |  |  |  | 0 | (2) | (1)
Net periodic benefit (credit) cost |  |  |  |  |  |  |  |  |  | 6 | 9 | 10
Curtailment (gain) loss |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Plan termination |  |  |  |  |  |  |  |  |  | 0 | 0 | (128)
Settlement loss |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total (credit) cost |  |  |  |  |  |  |  |  | $ (125) | 6 | 9 | (118)
Domestic Plan | Pension Plan |  |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Service cost |  |  |  |  |  |  |  |  |  | 0 | 106 | 114
Interest cost |  |  |  |  |  |  |  |  |  | 264 | 264 | 254
Expected return on plan assets |  |  |  |  |  |  |  |  |  | (357) | (379) | (373)
Amortization of prior service (credit) cost |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Recognized actuarial loss (gain) |  |  |  |  |  |  |  |  |  | 37 | 74 | 146
Net periodic benefit (credit) cost |  |  |  |  |  |  |  |  |  | (56) | 65 | 141
Plan termination |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total (credit) cost |  |  |  |  |  |  |  |  |  | (56) | 65 | 141
Domestic Plan | Postretirement Benefits |  |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Service cost |  |  |  |  |  |  |  |  |  | 0 | 0 | 1
Interest cost |  |  |  |  |  |  |  |  |  | 2 | 2 | 2
Expected return on plan assets |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization of prior service (credit) cost |  |  |  |  |  |  |  |  |  | 3 | 4 | 3
Recognized actuarial loss (gain) |  |  |  |  |  |  |  |  |  | (1) | (2) | (2)
Net periodic benefit (credit) cost |  |  |  |  |  |  |  |  |  | 4 | 4 | 4
Plan termination |  |  |  |  |  |  |  |  |  | 0 | 0 | (128)
Total (credit) cost |  |  |  |  |  |  |  |  |  | 4 | 4 | (124)
Foreign Plan | Pension Plan |  |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Service cost |  |  |  |  |  |  |  |  |  | 76 | 72 | 82
Interest cost |  |  |  |  |  |  |  |  |  | 233 | 273 | 333
Expected return on plan assets |  |  |  |  |  |  |  |  |  | (564) | (561) | (604)
Amortization of prior service (credit) cost |  |  |  |  |  |  |  |  |  | (2) | (1) | (1)
Recognized actuarial loss (gain) |  |  |  |  |  |  |  |  |  | 130 | 94 | 125
Net periodic benefit (credit) cost |  |  |  |  |  |  |  |  |  | (127) | (123) | (65)
Curtailment (gain) loss |  |  |  |  |  |  |  |  |  | (1) | (4) | 5
Settlement loss |  |  |  |  |  |  |  |  |  | 54 | 0 | 1
Total (credit) cost |  |  |  |  |  |  |  |  |  | (74) | (127) | (59)
Foreign Plan | Postretirement Benefits |  |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Service cost |  |  |  |  |  |  |  |  |  | 1 | 2 | 2
Interest cost |  |  |  |  |  |  |  |  |  | 2 | 3 | 3
Expected return on plan assets |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization of prior service (credit) cost |  |  |  |  |  |  |  |  |  | (2) | 0 | 0
Recognized actuarial loss (gain) |  |  |  |  |  |  |  |  |  | 1 | 0 | 1
Net periodic benefit (credit) cost |  |  |  |  |  |  |  |  |  | 2 | 5 | 6
Curtailment (gain) loss |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Settlement loss |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total (credit) cost |  |  |  |  |  |  |  |  |  | $ 2 | $ 5 | $ 6

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