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Employee Bargaining Agreements and Benefit Plans - Plan's Assets Carried at Fair Value (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | $ 11 | $ 16
Annuity Contract [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | 6 |
Other [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | 5 | 5
Equity Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements |  | 6
Fixed Income Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements |  | 5
Level 1 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | 1 | 12
Level 1 [Member] | Other [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | 1 | 1
Level 1 [Member] | Equity Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements |  | 6
Level 1 [Member] | Fixed Income Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements |  | 5
Level 2 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | 4 | 4
Level 2 [Member] | Other [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | 4 | $ 4
Level 3 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | 6 |
Level 3 [Member] | Annuity Contract [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total fair value measurements | $ 6 |

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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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