Financings and Capitalization (Summary of Long-Term Debt Outstanding) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Debt Instrument [Line Items]
Principal amounts outstanding	$ 8,547,000,000		$ 7,654,000,000
Current portion of long-term debt	(519,000,000)		(541,000,000)
Net unamortized discount	(12,000,000)		(12,000,000)
Total long-term debt	8,016,000,000		7,101,000,000
Debt retirement, principal	707,000,000
Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal amounts outstanding	5,283,000,000		4,625,000,000
Current portion of long-term debt	(124,000,000)		(43,000,000)
Net unamortized discount	(5,000,000)		(3,000,000)
Total long-term debt	5,154,000,000		4,579,000,000
Debt retirement, principal	177,000,000
CMS Energy [Member]
Debt Instrument [Line Items]
Principal amounts outstanding	2,380,000,000		2,377,000,000
Net unamortized discount	(7,000,000)		(9,000,000)
Total long-term debt	2,380,000,000		2,205,000,000
Debt retirement, principal	530,000,000
Senior Notes 6.875% Due December 2015 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	6.875%	[1]
Debt retirement, principal	125,000,000	[1]
Senior Notes 4.250% Due 2015 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	4.25%	[2]
Debt retirement, principal	250,000,000	[2]
Senior Notes 3.875% Due March 2024 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	3.875%
Senior Notes 5.500% Due June 2029 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	5.50%
Debt retirement, principal	155,000,000
Senior Notes 4.875% Due March 2044 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	4.875%
Term Loan Facility Due 2017 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Maturity Date	2017
Principal amounts outstanding	180,000,000		180,000,000
Three-month LIBOR plus a spread	1.25%
Interest rate at period end	1.42%
FMB's 5.500% Due August 2016 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	5.50%
Debt retirement, principal	177,000,000
FMB's 3.125% Due August 2024 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	3.125%	[3]
Maturity Date	2024	[3]
Principal amounts outstanding	250,000,000	[3]
FMB's 4.350% Due August 2064 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	4.35%
Securitization Bonds 5.760% Due 2015 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	5.76%
Maturity Date	2015
Principal amounts outstanding	49,000,000		92,000,000
Securitization Bonds 2.597% Due 2015-2029 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	2.597%	[4]
Maturity Date	2020-2029
Principal amounts outstanding	378,000,000
Securitization Bonds [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal amounts outstanding	427,000,000		92,000,000
Securitization Bonds [Member] | Consumers Energy Company [Member] | Consumers Funding [Member]
Debt Instrument [Line Items]
Weighted-average Interest Rate (%)	5.76%
Securitization Bonds [Member] | Consumers Energy Company [Member] | Consumers 2014 Securitization Funding [Member]
Debt Instrument [Line Items]
Weighted-average Interest Rate (%)	2.60%
Senior Notes 6.875% Due 2018 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	6.875%
Maturity Date	2018
Principal amounts outstanding	180,000,000		180,000,000
Tax-Exempt Pollution Control Revenue Bonds [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Maturity Date	2018-2035
Principal amounts outstanding	103,000,000		103,000,000
Senior Notes [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Principal amounts outstanding	2,200,000,000		2,197,000,000
Senior Notes [Member] | Senior Notes 6.875% Due December 2015 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	6.875%	[5]
Maturity Date	2015
Principal amounts outstanding			125,000,000
Senior Notes [Member] | Senior Notes 4.250% Due 2015 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	4.25%	[6]
Maturity Date	2015
Principal amounts outstanding			250,000,000
Senior Notes [Member] | Senior Notes 6.550% Due 2017 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	6.55%
Maturity Date	2017
Principal amounts outstanding	250,000,000		250,000,000
Senior Notes [Member] | Senior Notes 5.050% Due 2018 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	5.05%
Maturity Date	2018
Principal amounts outstanding	250,000,000		250,000,000
Senior Notes [Member] | Senior Notes 8.750% Due 2019 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	8.75%
Maturity Date	2019
Principal amounts outstanding	300,000,000		300,000,000
Senior Notes [Member] | Senior Notes 6.250% Due 2020 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	6.25%
Maturity Date	2020
Principal amounts outstanding	300,000,000		300,000,000
Senior Notes [Member] | Senior Notes 5.050% Due 2022 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	5.05%
Maturity Date	2022
Principal amounts outstanding	300,000,000		300,000,000
Senior Notes [Member] | Senior Notes 3.875% Due March 2024 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	3.875%
Maturity Date	2024
Principal amounts outstanding	250,000,000
Senior Notes [Member] | Senior Notes 5.500% Due June 2029 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	5.50%	[7]
Maturity Date	2029
Principal amounts outstanding			172,000,000
Senior Notes [Member] | Senior Notes 4.700% Due 2043 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	4.70%
Maturity Date	2043
Principal amounts outstanding	250,000,000		250,000,000
Senior Notes [Member] | Senior Notes 4.875% Due March 2044 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate	4.875%
Maturity Date	2044
Principal amounts outstanding	300,000,000
Other CMS Subsidiaries [Member]
Debt Instrument [Line Items]
Principal amounts outstanding	884,000,000		652,000,000
Other CMS Subsidiaries [Member] | EnerBank Certificates Of Deposit [Member]
Debt Instrument [Line Items]
Interest rate	1.218%	[8]
Maturity Date	2015-2024
Interest-bearing Domestic Deposit, Brokered	884,000,000		652,000,000
Weighted-average interest rate	1.22%		1.09%
Certificate of deposit face value	1,000
First Mortgage Bonds [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal amounts outstanding	4,573,000,000	[3]	4,250,000,000	[3]
Weighted-average Interest Rate (%)	4.75%		4.90%
First Mortgage Bonds [Member] | FMB's 2.600% Due 2015 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	2.60%	[3],[9]
Maturity Date	2015	[3]
Principal amounts outstanding	50,000,000	[3]	50,000,000	[3]
First Mortgage Bonds [Member] | FMB's 5.500% Due August 2016 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	5.50%	[3],[9]
Maturity Date	2016	[3]
Principal amounts outstanding	173,000,000	[3]	350,000,000	[3]
First Mortgage Bonds [Member] | FMB's 5.150% Due 2017 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	5.15%	[3]
Maturity Date	2017	[3]
Principal amounts outstanding	250,000,000	[3]	250,000,000	[3]
First Mortgage Bonds [Member] | FMB's 3.210% Due 2017 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	3.21%	[3]
Maturity Date	2017	[3]
Principal amounts outstanding	100,000,000	[3]	100,000,000	[3]
First Mortgage Bonds [Member] | FMB's 5.650% Due 2018 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	5.65%	[3]
Maturity Date	2018	[3]
Principal amounts outstanding	250,000,000	[3]	250,000,000	[3]
First Mortgage Bonds [Member] | FMB's 6.125% Due 2019 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	6.125%	[3]
Maturity Date	2019	[3]
Principal amounts outstanding	350,000,000	[3]	350,000,000	[3]
First Mortgage Bonds [Member] | FMB's 6.700% Due 2019 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	6.70%	[3]
Maturity Date	2019	[3]
Principal amounts outstanding	500,000,000	[3]	500,000,000	[3]
First Mortgage Bonds [Member] | FMB's 5.650% Due 2020 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	5.65%	[3]
Maturity Date	2020	[3]
Principal amounts outstanding	300,000,000	[3]	300,000,000	[3]
First Mortgage Bonds [Member] | FMB's 3.770% Due 2020 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	3.77%	[3]
Maturity Date	2020	[3]
Principal amounts outstanding	100,000,000	[3]	100,000,000	[3]
First Mortgage Bonds [Member] | FMB's 5.300% Due 2022 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	5.30%	[3]
Maturity Date	2022	[3]
Principal amounts outstanding	250,000,000	[3]	250,000,000	[3]
First Mortgage Bonds [Member] | FMB's 2.850% Due 2022 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	2.85%	[3]
Maturity Date	2022	[3]
Principal amounts outstanding	375,000,000	[3]	375,000,000	[3]
First Mortgage Bonds [Member] | FMB's 3.375% Due 2023 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	3.375%	[3]
Maturity Date	2023	[3]
Principal amounts outstanding	325,000,000	[3]	325,000,000	[3]
First Mortgage Bonds [Member] | FMB's 3.190% Due 2024 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	3.19%	[3]
Maturity Date	2024	[3]
Principal amounts outstanding	52,000,000	[3]	52,000,000	[3]
First Mortgage Bonds [Member] | FMB's 3.390% Due 2027 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	3.39%	[3]
Maturity Date	2027	[3]
Principal amounts outstanding	35,000,000	[3]	35,000,000	[3]
First Mortgage Bonds [Member] | FMB's 5.800% Due 2035 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	5.80%	[3]
Maturity Date	2035	[3]
Principal amounts outstanding	175,000,000	[3]	175,000,000	[3]
First Mortgage Bonds [Member] | FMB's 6.170% Due 2040 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	6.17%	[3]
Maturity Date	2040	[3]
Principal amounts outstanding	50,000,000	[3]	50,000,000	[3]
First Mortgage Bonds [Member] | FMB's 4.970% Due 2040 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	4.97%	[3]
Maturity Date	2040	[3]
Principal amounts outstanding	50,000,000	[3]	50,000,000	[3]
First Mortgage Bonds [Member] | FMB's 4.310% Due 2042 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	4.31%	[3]
Maturity Date	2042	[3]
Principal amounts outstanding	263,000,000	[3]	263,000,000	[3]
First Mortgage Bonds [Member] | FMB's 3.950% Due 2043 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	3.95%	[3]
Maturity Date	2043	[3]
Principal amounts outstanding	425,000,000	[3]	425,000,000	[3]
First Mortgage Bonds [Member] | FMB's 4.350% Due August 2064 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Interest rate	4.35%	[3]
Maturity Date	2064	[3]
Principal amounts outstanding	$ 250,000,000	[3]

[1] CMS Energy retired this debt at a premium and recorded a loss on extinguishment of $13 million in other expense on its consolidated statements of income.

[2] CMS Energy retired this debt at a premium and recorded a loss on extinguishment of $7 million in other expense on its consolidated statements of income.

[3] The weighted-average interest rate for Consumers’ FMBs was 4.75 percent at December 31, 2014 and 4.90 percent at December 31, 2013.

[4] The interest rate for Consumers’ Securitization bonds issued through its subsidiary Consumers Funding was 5.76 percent at December 31, 2014 and 2013.The weighted-average interest rate for Consumers’ Securitization bonds issued through its subsidiary Consumers 2014 Securitization Funding was 2.60 percent at December 31, 2014.Principal and interest payments are made semiannually beginning in 2015.

[5] In April 2014, CMS Energy retired its 6.875 percent senior notes.

[6] In December 2014, CMS Energy retired its 4.25 percent senior notes.

[7] In June 2014, CMS Energy retired its remaining 5.50 percent contingently convertible notes. See the “Contingently Convertible Securities” section in this Note for further discussion of the conversions.

[8] The weighted-average interest rate for EnerBank’s certificates of deposit was 1.22 percent at December 31, 2014 and 1.09 percent at December 31, 2013. EnerBank’s primary deposit product consists of brokered certificates of deposit with varying maturities and having a face value of $1,000.

[9] In August 2014, Consumers retired $177 million of its 5.50 percent FMBs.