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Retirement Benefits (Schedule of Plan Contributions) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Postretirement benefits contributions | $ 72 |  | $ 323 |  | $ 463
Consumers Energy Company [Member] |  |  |  |  |
Postretirement benefits contributions | 68 |  | 315 |  | 447
Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |  |  |
Postretirement benefits contributions | 65 | [1] | 67 | [1] |
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year | 74 |  |  |  |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Postretirement benefits contributions | 64 | [1] | 66 | [1] |
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year | 73 |  |  |  |
Pension Plans, Defined Benefit [Member] |  |  |  |  |
Postretirement benefits contributions |  |  | 250 | [2] |
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year | 50 |  |  |  |
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Postretirement benefits contributions |  |  | 245 | [2] |
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year | 49 |  |  |  |
VEBA Trust [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |  |  |
Postretirement benefits contributions | 45 | [1] | 48 | [1] |
VEBA Trust [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Postretirement benefits contributions | 45 | [1] | 47 | [1] |
401 (h) Component [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |  |  |
Postretirement benefits contributions | 20 | [1] | 19 | [1] |
401 (h) Component [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Postretirement benefits contributions | $ 19 | [1] | $ 19 | [1] |
Equity Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 50.00% |  |  |  |
Equity Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 50.00% |  |  |  |
Equity Securities [Member] | Pension Plans, Defined Benefit [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 50.00% |  | 50.00% |  |
Equity Securities [Member] | Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 50.00% |  | 50.00% |  |
Fixed Income Funds [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 20.00% |  |  |  |
Fixed Income Funds [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 20.00% |  |  |  |
Fixed Income Funds [Member] | Pension Plans, Defined Benefit [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 30.00% |  | 30.00% |  |
Fixed Income Funds [Member] | Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 30.00% |  | 30.00% |  |
Alternative Strategy Investments [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 30.00% |  |  |  |
Alternative Strategy Investments [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 30.00% |  |  |  |
Alternative Strategy Investments [Member] | Pension Plans, Defined Benefit [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 20.00% |  | 20.00% |  |
Alternative Strategy Investments [Member] | Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 20.00% |  | 20.00% |  |
[1] | CMS Energy, including Consumers, plans to contribute $74 million to the OPEB plan in 2013, of which Consumers plans to contribute $73 million.
[2] | CMS Energy, including Consumers, planned to contribute $50 million to the Pension Plan in 2013, of which Consumers planned to contribute $49 million. This Pension Plan contribution was made in January 2013.

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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + 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 (g) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph g -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Target allocation of investments of this type to total plan assets presented on a weighted average basis. + 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 (d)(i) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e2446-114920 + Details
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- Definition The amount of cash or cash equivalents contributed during the reporting period by the entity to fund its pension plans and its non-pension postretirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3536-108585 + Details
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