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FAIR VALUE MEASUREMENTS (Tables) | 12 Months Ended
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Dec. 31, 2017
Fair Value Disclosures [Abstract] |
Schedule of Realized Gain (Loss) [Table Text Block] | Activity for marketable securities (in millions):
| 2017 |  | 2016 |  | 2015
Proceeds from sales and maturities | $ | 1,388 |  |  | $ | 962 |  |  | $ | 1,175 |
Schedule of Debt Investment Maturities [Table Text Block] | Maturities for marketable securities (in millions):
December 31, 2017 | Cost Basis |  | Fair Value
Due in one year or less | $ | 113 |  |  | $ | 113 |
Due after one year through five years | 1,272 |  |  | 1,264 |
Due after five years through 10 years | 50 |  |  | 50 |
Total | $ | 1,435 |  |  | $ | 1,427 |
Fair Value, by Balance Sheet Grouping [Table Text Block] | Fair values of financial instruments on the consolidated balance sheet (in millions):
December 31, 2017 | Level 1 |  | Level 2 |  | Total
Assets |  |  |  |  |
Marketable securities |  |  |  |  |
U.S. government and agency securities | $ | 328 |  |  | $ | — |  |  | $ | 328 |
Foreign government bonds | — |  |  | 43 |  |  | 43 |
Asset-backed securities | — |  |  | 209 |  |  | 209 |
Mortgage-backed securities | — |  |  | 99 |  |  | 99 |
Corporate notes and bonds | — |  |  | 726 |  |  | 726 |
Municipal securities | — |  |  | 22 |  |  | 22 |
Derivative instruments |  |  |  |  |
Fuel hedge contracts—call options | — |  |  | 22 |  |  | 22 |
Interest rate swap agreements | — |  |  | 9 |  |  | 9 |
Liabilities |  |  |  |  |
Derivative instruments |  |  |  |  |
Interest rate swap agreements | — |  |  | (8 | ) |  | (8 | )
December 31, 2016 | Level 1 |  | Level 2 |  | Total
Assets |  |  |  |  |
Marketable securities |  |  |  |  |
U.S. government and agency securities | $ | 287 |  |  | $ | — |  |  | $ | 287 |
Foreign government bonds | — |  |  | 36 |  |  | 36 |
Asset-backed securities | — |  |  | 138 |  |  | 138 |
Mortgage-backed securities | — |  |  | 89 |  |  | 89 |
Corporate notes and bonds | — |  |  | 691 |  |  | 691 |
Municipal securities | — |  |  | 11 |  |  | 11 |
Derivative instruments |  |  |  |  |
Fuel hedge contracts—call options | — |  |  | 20 |  |  | 20 |
Liabilities |  |  |  |  |
Derivative instruments |  |  |  |  |
Interest rate swap agreements | — |  |  | (5 | ) |  | (5 | )
Fixed-rate debt on the consolidated balance sheet and the estimated fair value of long-term fixed-rate debt (in millions):
| 2017 |  | 2016
Fixed rate debt at cost | $ | 956 |  |  | $ | 1,175 |
Non-recurring purchase price accounting fair value adjustment | 3 |  |  | 4 |
Total fixed rate debt | $ | 959 |  |  | $ | 1,179 |
December 31, 2017 estimated fair value | $ | 959 |  |  | $ | 1,199 |

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- Definition Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13476-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13467-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6447952&loc=d3e13220-108610 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14172-108612 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611 + Details
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- Definition Tabular disclosure of the proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses that have been included in earnings as a result of those sales. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563 + Details
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