Summary of Significant Accounting Policies (Accumulated Other Comprehensive Income) (Details) - USD ($)
|
12 Months Ended |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
| Accumulated Other Comprehensive Income (Loss), Pension and PRB Plans, Net - Beginning balance |
$ (7,843,000,000)
|
$ (7,234,000,000)
|
$ (7,088,000,000)
|
| Pension and PRB plans, net - Before tax amount |
1,028,000,000
|
(651,000,000)
|
(203,000,000)
|
| Pension and PRB plans, net - Tax (expense) benefit |
(216,000,000)
|
42,000,000
|
57,000,000
|
| Pension and PRB plans, net - Net of tax amount |
812,000,000
|
(609,000,000)
|
(146,000,000)
|
| Pension and PRB plans, net, Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings |
(1,452,000,000)
|
|
|
| Accumulated Other Comprehensive Income (Loss), Pension and PRB Plans, Net - Ending balance |
(8,483,000,000)
|
(7,843,000,000)
|
(7,234,000,000)
|
| Accumulated Other Comprehensive Income (Loss), Foreign Exchange Translation - Beginning balance |
(95,000,000)
|
(175,000,000)
|
(60,000,000)
|
| Foreign exchange translation - Before tax amount |
(36,000,000)
|
80,000,000
|
(115,000,000)
|
| Foreign exchange translation - Tax (expense) benefit |
0
|
0
|
0
|
| Foreign exchange translation - Net of tax amount |
(36,000,000)
|
80,000,000
|
(115,000,000)
|
| Foreign exchange translation, Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings |
0
|
|
|
| Accumulated Other Comprehensive Income (Loss), Foreign Exchange Translation - Ending balance |
(131,000,000)
|
(95,000,000)
|
(175,000,000)
|
| Accumulated Other Comprehensive Income (Loss), Cash flow hedges - Beginning balance |
6,000,000
|
0
|
(16,000,000)
|
| Cash flow hedges - Before tax amount |
(12,000,000)
|
10,000,000
|
25,000,000
|
| Cash flow hedges - Tax (expense) benefit |
3,000,000
|
(4,000,000)
|
(9,000,000)
|
| Cash flow hedges - Net of tax amount |
(9,000,000)
|
6,000,000
|
16,000,000
|
| Cash flow hedges, Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings |
1,000,000
|
|
|
| Accumulated Other Comprehensive Income (Loss), Cash flow hedges - Ending balance |
(2,000,000)
|
6,000,000
|
0
|
| Accumulated Other Comprehensive Income (Loss), Unrealized gains (losses) on investments, and other, net - Beginning balance |
(3,000,000)
|
(2,000,000)
|
(12,000,000)
|
| Unrealized gains (losses) on investments and other, net - Before tax amount |
1,000,000
|
(1,000,000)
|
15,000,000
|
| Unrealized gains (losses) on investments and other, net - tax (expense) benefit |
0
|
0
|
(5,000,000)
|
| Unrealized gains (losses) on investments and other, net, net of tax amount |
1,000,000
|
(1,000,000)
|
10,000,000
|
| Unrealized gains (losses) on investments and other, net, Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings |
0
|
|
|
| Accumulated Other Comprehensive Income (Loss), Unrealized gains (losses) on investments, and other, net - Ending balance |
(2,000,000)
|
(3,000,000)
|
(2,000,000)
|
| Accumulated other comprehensive income (loss), net of tax, - Beginning balance |
(7,935,000,000)
|
(7,411,000,000)
|
(7,176,000,000)
|
| Other comprehensive income (loss) - Before tax amount |
981,000,000
|
(562,000,000)
|
(278,000,000)
|
| Other comprehensive income (loss) - Tax (expense) benefit |
(213,000,000)
|
38,000,000
|
43,000,000
|
| Other comprehensive income (loss), net of tax |
768,000,000
|
(524,000,000)
|
(235,000,000)
|
| Reclassification of stranded tax effects |
(1,451,000,000)
|
0
|
0
|
| Accumulated other comprehensive income (loss) net of tax, - Ending balance |
(8,618,000,000)
|
(7,935,000,000)
|
$ (7,411,000,000)
|
| Pension and PRB plans, net - cumulative tax benefit |
2,255,000,000
|
3,923,000,000
|
|
| Cash flow hedges - cumulative tax benefit (expense) |
1,000,000
|
(3,000,000)
|
|
| Unrealized gains (losses) on investments and other, net - cumulative tax benefit (expense) |
$ (1,000,000)
|
$ (1,000,000)
|
|