Fair Value Measurements (Assets And Liabilties Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 349	[1]	$ 110	[1]
Restricted cash equivalents	50	[1]	38	[1]
Nonqualified deferred compensation plan assets	9	[1]	8	[1]
Commodity contracts	1	[1]	2	[1]
Total	563	[1]	289	[1]
Nonqualified deferred compensation plan liabilities	9	[1]	8	[1]
Commodity contracts	1	[1]	1	[1]
Total	10	[1]	9	[1]
Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	309	[1]	19	[1]
Restricted cash equivalents	50	[1]	38	[1]
Nonqualified deferred compensation plan assets	6	[1]	6	[1]
Commodity contracts	1	[1]	2	[1]
Total	503	[1]	196	[1]
Nonqualified deferred compensation plan liabilities	6	[1]	6	[1]
Commodity contracts			1	[1]
Total	6	[1]	7	[1]
DB SERP [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3	[1]	4	[1]
DB SERP [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2	[1]	3	[1]
DB SERP [Member] | Mutual Fund [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale - Fair Value	151	[1]	127	[1]
DB SERP [Member] | Mutual Fund [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale - Fair Value	107	[1]	90	[1]
CMS Energy Common Stock [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale - Fair Value	$ 28	[1]	$ 38	[1]

[1] All assets and liabilities were classified as Level 1 with the exception of some commodity contracts, which were classified as Level 2 or Level 3 and which were insignificant at March 31, 2015 and December 31, 2014.