Fair Value Measurements (Assets And Liabilties Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Cash equivalents	$ 339	$ 53
Restricted cash equivalents	13	14
Nonqualified deferred compensation plan assets	5	5
Commodity contracts	12	3
Total	507	203
Nonqualified deferred compensation plan liabilities	5	5
Commodity contracts	4	4
Total	9	9
Consumers Energy Company [Member]
Cash equivalents	186
Restricted cash equivalents	13	13
Nonqualified deferred compensation plan assets	4	4
Commodity contracts	10	2
Total	340	137
Nonqualified deferred compensation plan liabilities	4	4
Total	4	4
Fair Value, Inputs, Level 1 [Member]
Cash equivalents	339	53
Restricted cash equivalents	13	14
Nonqualified deferred compensation plan assets	5	5
Total	495	200
Nonqualified deferred compensation plan liabilities	5	5
Total	5	5
Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member]
Cash equivalents	186
Restricted cash equivalents	13	13
Nonqualified deferred compensation plan assets	4	4
Total	330	135
Nonqualified deferred compensation plan liabilities	4	4
Total	4	4
Fair Value, Inputs, Level 2 [Member]
Commodity contracts	2
Total	2
Commodity contracts	2	3
Total	2	3
Fair Value, Inputs, Level 3 [Member]
Commodity contracts	10	3
Total	10	3
Commodity contracts	2	1
Total	2	1
Fair Value, Inputs, Level 3 [Member] | Consumers Energy Company [Member]
Commodity contracts	10	2
Total	10	2
Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
Available for sale - Fair Value	1	2
Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member] | Consumers Energy Company [Member]
Available for sale - Fair Value	1	1
Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
Available for sale - Fair Value	137	126
Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member] | Consumers Energy Company [Member]
Available for sale - Fair Value	96	85
Supplemental Executive Retirement Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]
Available for sale - Fair Value	1	2
Supplemental Executive Retirement Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member] | Consumers Energy Company [Member]
Available for sale - Fair Value	1	1
Supplemental Executive Retirement Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Mutual Fund [Member]
Available for sale - Fair Value	137	126
Supplemental Executive Retirement Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Mutual Fund [Member] | Consumers Energy Company [Member]
Available for sale - Fair Value	96	85
CMS Energy Common Stock [Member] | Consumers Energy Company [Member]
Available for sale - Fair Value	30	32
CMS Energy Common Stock [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member]
Available for sale - Fair Value	$ 30	$ 32