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Employee Benefit Plans (Net Periodic Benefit (Credit) Cost and Amounts Recognized in Other Comprehensive Income and Regulatory Assets and Liabilities) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Pension Benefits |  |  |  |
Service cost |  | $ 173 | $ 162 | $ 157
Interest cost |  | 351 | 394 | 337
Expected return on plan assets |  | (777) | (708) | (663)
Amortization of prior service (credit) cost |  | 1 | 1 | 1
Amortization of net actuarial loss |  | 206 | 172 | 193
Settlements, curtailments and special termination benefits |  | 14 | 72 |
Net periodic benefit (credit) cost |  | (32) | 93 | 25
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |
Current year net actuarial (gain) loss |  | 166 | 16 | 490
Settlements and curtailments |  | (81) | 6 |
Amortization of net actuarial loss |  | (206) | (172) | (193)
Amortization of prior service credit (cost) |  | (1) | (1) | (1)
Total recognized in other comprehensive income and regulatory assets and liabilities |  | $ (122) | $ (151) | $ 296
Significant assumptions used to determine periodic cost: |  |  |  |
Expected long-term rate of return on plan assets |  |  |  | 8.75%
Weighted average rate of increase for compensation |  | 4.23% | 4.20% | 4.09%
Pension Benefits | Minimum |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |
Discount rate |  | 2.77% | 3.57% | 3.80%
Expected long-term rate of return on plan assets |  | 7.00% | 7.00% |
Crediting interest rate for cash balance and similar plans |  | 2.31% | 2.77% | 3.00%
Pension Benefits | Maximum |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |
Discount rate |  | 3.63% | 4.43% | 3.81%
Expected long-term rate of return on plan assets |  | 8.60% | 8.65% |
Crediting interest rate for cash balance and similar plans |  | 2.83% | 3.63% | 3.01%
Other Postretirement Benefits |  |  |  |
Service cost |  | $ 28 | $ 26 | $ 27
Interest cost |  | 58 | 68 | 56
Expected return on plan assets |  | (156) | (140) | (143)
Amortization of prior service (credit) cost |  | (49) | (52) | (52)
Amortization of net actuarial loss |  | 6 | 10 | 11
Settlements, curtailments and special termination benefits |  | (59) | 42 |
Net periodic benefit (credit) cost |  | (172) | (46) | (101)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |
Current year net actuarial (gain) loss |  | (110) | (98) | 78
Prior service (credit) cost |  | (6) | 2 | (4)
Settlements and curtailments |  | 59 |  |
Amortization of net actuarial loss |  | (6) | (10) | (11)
Amortization of prior service credit (cost) |  | 49 | 52 | 52
Total recognized in other comprehensive income and regulatory assets and liabilities |  | $ (14) | $ (54) | $ 115
Significant assumptions used to determine periodic cost: |  |  |  |
Discount rate |  |  |  | 3.76%
Expected long-term rate of return on plan assets |  | 8.50% | 8.50% | 8.50%
Health care cost trend rate | [1] | 6.25% |  | 7.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) | [1] | 5.00% | 5.00% | 5.00%
Year that the rate reaches the ultimate trend rate | [1] |  |  | 2022
Other Postretirement Benefits | Minimum |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |
Discount rate |  | 3.07% | 4.05% |
Health care cost trend rate | [1] |  | 6.50% |
Year that the rate reaches the ultimate trend rate | [1] | 2025 | 2023 |
Other Postretirement Benefits | Maximum |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |
Discount rate |  | 3.52% | 4.41% |
Health care cost trend rate | [1] |  | 6.60% |
Year that the rate reaches the ultimate trend rate | [1] | 2026 | 2025 |
[1] | Assumptions used to determine net periodic cost for the following year.

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- Definition Other comprehensive income loss defined benefit plan amortization of prior service cost credit after reclassification adjustment before tax. + References No definition available. + Details
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- Definition Amount of (increase) decrease in accumulated benefit obligation of defined benefit plan from effects of settlement and curtailment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (r) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + 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 (k)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + 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 (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted-average interest crediting rate used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, cash balance and other defined benefit plans with promised interest crediting rate. + 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 (k)(4) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4587-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 3A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109237824&loc=SL108413299-114919 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 + Details
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- Definition Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status. + 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 (l) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit plan. + 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 (h)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 3A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109237824&loc=SL108413299-114919 Reference 3: 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 4: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4587-114921 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -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 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 + Details
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- Definition Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan. + 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 (h)(1) -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 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 5: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4587-114921 + Details
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