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Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
DB Pension Plans |  |  |  |
Net periodic cost (credit) |  |  |  |
Service cost | $ 12 | $ 11 | $ 24 | $ 22
Interest cost | 23 | 22 | 45 | 44
Expected return on plan assets | (37) | (38) | (74) | (76)
Amortization of: |  |  |  |
Net loss | 19 | 20 | 37 | 40
Prior service cost (credit) | 0 | 1 | 1 | 2
Net periodic cost (credit) | 17 | 16 | 33 | 32
DB Pension Plans | Consumers Energy Company |  |  |  |
Net periodic cost (credit) |  |  |  |
Service cost | 11 | 11 | 23 | 22
Interest cost | 21 | 22 | 42 | 43
Expected return on plan assets | (34) | (37) | (69) | (74)
Amortization of: |  |  |  |
Net loss | 18 | 19 | 36 | 39
Prior service cost (credit) | 0 | 1 | 1 | 2
Net periodic cost (credit) | 16 | 16 | 33 | 32
OPEB Plan |  |  |  |
Net periodic cost (credit) |  |  |  |
Service cost | 5 | 5 | 9 | 10
Interest cost | 9 | 13 | 18 | 26
Expected return on plan assets | (25) | (23) | (49) | (45)
Amortization of: |  |  |  |
Net loss | 4 | 8 | 8 | 16
Prior service cost (credit) | (17) | (9) | (34) | (18)
Net periodic cost (credit) | (24) | (6) | (48) | (11)
OPEB Plan | Consumers Energy Company |  |  |  |
Net periodic cost (credit) |  |  |  |
Service cost | 4 | 4 | 8 | 9
Interest cost | 9 | 13 | 17 | 26
Expected return on plan assets | (23) | (21) | (46) | (42)
Amortization of: |  |  |  |
Net loss | 4 | 8 | 8 | 16
Prior service cost (credit) | (16) | (8) | (32) | (17)
Net periodic cost (credit) | $ (22) | $ (4) | $ (45) | $ (8)

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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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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