Financial Instruments (Schedule of Cost of Carrying Amounts and Fair Value of Long-Term Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Securities held to maturity, Cost or Carrying Amount	$ 8		$ 7
Securities held to maturity - fair value	9		7
Securities available for sale, Cost or Carrying Amount	125		113
Securities available for sale, fair value	127		113
Long-term debt, Cost or Carrying Amount	6,852	[1]	7,073	[1]
Long-term Debt, Fair Value	7,812	[1]	8,025	[1]
Notes receivable	40		49
Current portion of long-term debt	631		1,033
Portion of long-term debt supported by third-party credit enhancements	103		103
Consumers Energy Company [Member]
Securities available for sale, Cost or Carrying Amount	90		81
Securities available for sale, fair value	116		109
Long-term debt, Cost or Carrying Amount	4,007	[2]	4,326	[2]
Long-term Debt, Fair Value	4,625	[2]	4,882	[2]
Notes receivable	13		23
Current portion of long-term debt	40		339
Portion of long-term debt supported by third-party credit enhancements	103		103
EnerBank USA [Member]
Total unsecured consumer installment loans, net of an allowance for loan losses	486	[3]	480	[3]
Notes receivable, Fair Value	513	[3]	504	[3]
Notes receivable	26		19
Fair Value, Inputs, Level 1 [Member]
Securities available for sale, fair value	127
Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member]
Securities available for sale, fair value	116
Long-term Debt, Fair Value		[2]
Fair Value, Inputs, Level 2 [Member]
Securities held to maturity - fair value	9
Long-term Debt, Fair Value	7,812	[1]
Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
Securities available for sale, fair value
Long-term Debt, Fair Value	4,625	[2]
Fair Value, Inputs, Level 3 [Member] | Consumers Energy Company [Member]
Securities available for sale, fair value
Long-term Debt, Fair Value		[2]
Fair Value, Inputs, Level 3 [Member] | EnerBank USA [Member]
Notes receivable, Fair Value	$ 513	[3]

[1] Includes current portion of long-term debt of $631 million at June 30, 2012 and $1,033 million at December 31, 2011.

[2] Includes current portion of long-term debt of $40 million at June 30, 2012 and $339 million at December 31, 2011.

[3] Includes current portion of notes receivable of $26 million at June 30, 2012 and $19 million at December 31, 2011.