Stock-Based Compensation (Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Arrangements Plan Term	10 years
Number of shares authorized	6,500,000
Maximum shares issuable per employee	500,000
Shares available for grant	4,342,829
Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Arrangements Plan Term	10 years
Number of shares authorized	6,500,000
Maximum shares issuable per employee	500,000
Shares available for grant	4,342,829
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of Initial Grant Issued on Vesting Date	0.00%
Minimum [Member] \| Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of Initial Grant Issued on Vesting Date	0.00%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of Initial Grant Issued on Vesting Date	200.00%
Maximum [Member] \| Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of Initial Grant Issued on Vesting Date	200.00%
Performance-Based Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	38 months
Vesting period	3 years
Performance-Based Restricted Stock [Member] \| Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	38 months
Vesting period	3 years
Performance-Based Restricted Stock [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	36 months
Performance-Based Restricted Stock [Member] \| Minimum [Member] \| Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	36 months
Market-Based Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	3 years
Vesting period	3 years
Market-Based Restricted Stock [Member] \| Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	3 years
Vesting period	3 years
Time-Lapsed Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	3 years
Time-Lapsed Restricted Stock [Member] \| Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	3 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	1 year
Shares forfeited	0
Restricted Stock Units (RSUs) [Member] \| Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period	1 year
Shares forfeited	0
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares forfeited	93,501
Unrecognized compensation cost \| $	$ 18
Unrecognized compensation cost recognition period	2 years
Restricted Stock [Member] \| Consumers Energy Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares forfeited	84,293
Unrecognized compensation cost \| $	$ 17
Unrecognized compensation cost recognition period	2 years

Deferred Compensation Arrangements Plan Term

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- Definition

Percentage of initial grant of equity-based compensation awards earned depending on the outcome of the market condition or performance condition.

+ References

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- Definition

The highest quantity of shares an employee can be awarded under the plan per period.

+ References

No definition available.

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
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- Definition

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

+ References

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

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