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Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
DB Pension Plans and DB SERP |  |  |
Defined Benefit Plan, Roll Forwards [Abstract] |  |  |
Service cost | $ 41 | $ 48 | $ 45
Interest cost | 103 | 95 | 93
Expected return on plan assets | (162) | (149) | (153)
Amortization of |  |  |
Net loss | 50 | 76 | 82
Prior service cost (credit) | 1 | 3 | 5
Net periodic cost (credit) | 33 | 73 | 72
DB Pension Plans and DB SERP | Consumers Energy Company |  |  |
Defined Benefit Plan, Roll Forwards [Abstract] |  |  |
Service cost | 40 | 47 | 44
Interest cost | 97 | 88 | 90
Expected return on plan assets | (153) | (139) | (149)
Amortization of |  |  |
Net loss | 47 | 73 | 79
Prior service cost (credit) | 1 | 3 | 4
Net periodic cost (credit) | 32 | 72 | 68
OPEB Plan |  |  |
Defined Benefit Plan, Roll Forwards [Abstract] |  |  |
Service cost | 14 | 17 | 19
Interest cost | 41 | 34 | 51
Expected return on plan assets | (88) | (97) | (90)
Amortization of |  |  |
Net loss | 26 | 15 | 29
Prior service cost (credit) | (62) | (67) | (40)
Net periodic cost (credit) | (69) | (98) | (31)
OPEB Plan | Consumers Energy Company |  |  |
Defined Benefit Plan, Roll Forwards [Abstract] |  |  |
Service cost | 13 | 16 | 19
Interest cost | 40 | 33 | 49
Expected return on plan assets | (82) | (91) | (84)
Amortization of |  |  |
Net loss | 26 | 16 | 29
Prior service cost (credit) | (61) | (65) | (39)
Net periodic cost (credit) | $ (64) | $ (91) | $ (26)

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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit 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 (h)(4) -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)(4) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 + Details
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