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Financial Instruments (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Financial Instruments [Line Items] |  |  |
Portion of long-term debt supported by third-party credit enhancements |  | $ 35 |
Proceeds from DB SERP investments | $ 0 | 0 | $ 146
Consumers Energy Company |  |  |
Financial Instruments [Line Items] |  |  |
Proceeds from DB SERP investments | $ 0 | $ 0 | 106
DB SERP |  |  |
Financial Instruments [Line Items] |  |  |
Proceeds from DB SERP investments |  |  | 142
DB SERP | Consumers Energy Company |  |  |
Financial Instruments [Line Items] |  |  |
Proceeds from DB SERP investments |  |  | $ 103

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- Definition Financial Instruments [Line Items] + References No definition available. + Details
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- Definition Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26853-111562 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3151-108585 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3179-108585 + Details
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- Details
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