Financial Instruments (Schedule of Cost of Carrying Amounts and Fair Value of Long-Term Financial Instruments) (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Securities held to maturity, Cost or Carrying Amount	$ 7		$ 5
Securities held to maturity - fair value	8		6
Securities available for sale, Cost or Carrying Amount	113		90
Securities available for sale, fair value	113		90
Total unsecured consumer installment loans, net of an allowance for loan losses	439	[1]	386	[1]
Notes receivable, Fair Value	459	[1]	407	[1]
Long-term debt, Cost or Carrying Amount	7,153	[2]	7,174	[2]
Long-term Debt, Fair Value	8,093	[2]	7,861	[2]
Notes receivable	62		70
Current portion of long-term debt	1,116		726
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	81		64
Securities available for sale, fair value	105		90
Long-term debt, Cost or Carrying Amount	4,335	[3]	4,525	[3]
Long-term Debt, Fair Value	4,901	[3]	4,891	[3]
Notes receivable	43		55
Current portion of long-term debt	338		37
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	439		386
Notes receivable	$ 16		$ 11

[1] 1 Includes current portion of notes receivable of $16 million at September 30, 2011 and $11 million at December 31, 2010.

[2] 2 Includes current portion of long-term debt of $1,116 million at September 30, 2011 and $726 million at December 31, 2010.

[3] 3 Includes current portion of long-term debt of $338 million at September 30, 2011 and $37 million at December 31, 2010.