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Other Comprehensive Income (Loss) - Changes in AOCI by Component (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Beginning Balance | $ 40,308 | $ 44,767 | $ 48,791
Accumulated Other Comprehensive Income (Loss), ending balance | (4,910) | (5,226) |
Ending Balance | 34,569 | 40,308 | 44,767
Derivatives |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Beginning Balance | 338 | 404 | 530
Other comprehensive income (loss) before reclassification adjustments, pretax | (561) | 210 | 526
Tax | 207 | (72) | (177)
Other comprehensive income (loss) before reclassification adjustments, net of taxes | (354) | 138 | 349
Reclassification adjustments, pretax | (141) | (314) | (731)
Tax | 49 | 110 | 256
Reclassification adjustments, net of taxes | (92) | (204) | (475)
Other comprehensive income (loss), net of taxes | (446) | (66) | (126)
Ending Balance | (108) | 338 | 404
Investments |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Beginning Balance | (3) | 41 | 111
Other comprehensive income (loss) before reclassification adjustments, pretax | 212 | (38) | (9)
Tax | (35) | 16 | (13)
Other comprehensive income (loss) before reclassification adjustments, net of taxes | 177 | (22) | (22)
Reclassification adjustments, pretax | (291) | (31) | (73)
Tax | 56 | 9 | 25
Reclassification adjustments, net of taxes | (235) | (22) | (48)
Other comprehensive income (loss), net of taxes | (58) | (44) | (70)
Ending Balance | (61) | (3) | 41
Employee Benefit Plans |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Beginning Balance | (3,206) | (2,407) | (2,986)
Other comprehensive income (loss) before reclassification adjustments, pretax | 438 | (1,199) | 710
Tax | (106) | 363 | (272)
Other comprehensive income (loss) before reclassification adjustments, net of taxes | 332 | (836) | 438
Reclassification adjustments, pretax | 117 | 37 | 203
Tax | (30) | 0 | (62)
Reclassification adjustments, net of taxes | 87 | 37 | 141
Other comprehensive income (loss), net of taxes | 419 | (799) | 579
Ending Balance | (2,787) | (3,206) | (2,407)
Pension Plan Net Loss |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Pension and other postretirement benefit plans, net loss and prior service credit included in AOCI | 3,500 | 3,900 |
Other Postretirement Benefit Plan Net Loss |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Pension and other postretirement benefit plans, net loss and prior service credit included in AOCI | (16) | 115 |
Pension Plan Prior Service Credit |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Pension and other postretirement benefit plans, net loss and prior service credit included in AOCI | 326 | 361 |
Other Postretirement Benefit Plan Prior Service Credit |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Pension and other postretirement benefit plans, net loss and prior service credit included in AOCI | 383 | 466 |
Cumulative Translation Adjustment |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Beginning Balance | (2,355) | (2,186) | (1,978)
Other comprehensive income (loss) before reclassification adjustments, pretax | 235 | (150) | (158)
Tax | 166 | (19) | (28)
Other comprehensive income (loss) before reclassification adjustments, net of taxes | 401 | (169) | (186)
Reclassification adjustments, pretax | 0 | 0 | (22)
Tax | 0 | 0 | 0
Reclassification adjustments, net of taxes | 0 | 0 | (22)
Other comprehensive income (loss), net of taxes | 401 | (169) | (208)
Ending Balance | (1,954) | (2,355) | (2,186)
Accumulated Other Comprehensive Income (Loss) |  |  |
Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Beginning Balance | (5,226) | (4,148) | (4,323)
Other comprehensive income (loss) before reclassification adjustments, pretax | 324 | (1,177) | 1,069
Tax | 232 | 288 | (490)
Other comprehensive income (loss) before reclassification adjustments, net of taxes | 556 | (889) | 579
Reclassification adjustments, pretax | (315) | (308) | (623)
Tax | 75 | 119 | 219
Reclassification adjustments, net of taxes | (240) | (189) | (404)
Other comprehensive income (loss), net of taxes | 316 | (1,078) | 175
Ending Balance | $ (4,910) | $ (5,226) | $ (4,148)

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Amount, after tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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- Definition Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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