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Financings and Capitalization (Summary of Long-Term Debt Outstanding) (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014
Debt Instrument [Line Items] |  |  |
Principal amounts outstanding |  | $ 9,137,000,000 | $ 8,547,000,000
Current portion of long-term debt |  | (684,000,000) | (519,000,000)
Net unamortized discounts |  | (12,000,000) | (12,000,000)
Total long-term debt |  | 8,441,000,000 | 8,016,000,000
Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Principal amounts outstanding |  | 5,409,000,000 | 5,283,000,000
Current portion of long-term debt |  | (198,000,000) | (124,000,000)
Net unamortized discounts |  | (5,000,000) | (5,000,000)
Total long-term debt |  | 5,206,000,000 | 5,154,000,000
CMS Energy [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Principal amounts outstanding |  | 2,630,000,000 | 2,380,000,000
Net unamortized discounts |  | (7,000,000) | (7,000,000)
Total long-term debt |  | $ 2,630,000,000 | 2,380,000,000
Senior Notes 3.600% Due March 2025 [Member] | CMS Energy [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate |  | 3.60% |
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 |  | 0.85% |
Interest rate at period end |  | 1.19% |
FMB's 2.600% Due 2015 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate |  | 2.60% |
FMB's 4.100% Due 2045 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate |  | 4.10% |
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 |  | $ 2,450,000,000 | 2,200,000,000
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 June 2024 [Member] | CMS Energy [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate |  | 3.875% |
Maturity Date |  | 2024 |
Principal amounts outstanding |  | $ 250,000,000 | 250,000,000
Senior Notes [Member] | Senior Notes 3.600% Due March 2025 [Member] | CMS Energy [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate |  | 3.60% |
Maturity Date |  | 2025 |
Principal amounts outstanding |  | $ 250,000,000 |
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 | 300,000,000
Senior Notes [Member] | 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
Other CMS Subsidiaries [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Principal amounts outstanding |  | $ 1,098,000,000 | 884,000,000
Other CMS Subsidiaries [Member] | EnerBank Certificates Of Deposit [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate |  | 1.365% |
Maturity Date |  | 2016-2025 |
Interest-bearing Domestic Deposit, Brokered |  | $ 1,098,000,000 | $ 884,000,000
Weighted-average interest rate |  | 1.36% | 1.22%
Certificate of deposit face value |  | $ 1,000 |
First Mortgage Bonds [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Principal amounts outstanding |  | $ 4,773,000,000 | $ 4,573,000,000
Weighted-average Interest Rate (%) |  | 4.73% | 4.75%
First Mortgage Bonds [Member] | FMB's 2.600% Due 2015 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 2.60% |
Maturity Date | [1] | 2015 |
Principal amounts outstanding | [1] |  | $ 50,000,000
First Mortgage Bonds [Member] | FMB's 5.500% Due August 2016 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 5.50% |
Maturity Date | [1] | 2016 |
Principal amounts outstanding | [1] | $ 173,000,000 | 173,000,000
First Mortgage Bonds [Member] | FMB's 5.150% Due 2017 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 5.15% |
Maturity Date | [1] | 2017 |
Principal amounts outstanding | [1] | $ 250,000,000 | 250,000,000
First Mortgage Bonds [Member] | FMB's 3.210% Due 2017 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 3.21% |
Maturity Date | [1] | 2017 |
Principal amounts outstanding | [1] | $ 100,000,000 | 100,000,000
First Mortgage Bonds [Member] | FMB's 5.650% Due 2018 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 5.65% |
Maturity Date | [1] | 2018 |
Principal amounts outstanding | [1] | $ 250,000,000 | 250,000,000
First Mortgage Bonds [Member] | FMB's 6.125% Due 2019 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 6.125% |
Maturity Date | [1] | 2019 |
Principal amounts outstanding | [1] | $ 350,000,000 | 350,000,000
First Mortgage Bonds [Member] | FMB's 6.700% Due 2019 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 6.70% |
Maturity Date | [1] | 2019 |
Principal amounts outstanding | [1] | $ 500,000,000 | 500,000,000
First Mortgage Bonds [Member] | FMB's 5.650% Due 2020 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 5.65% |
Maturity Date | [1] | 2020 |
Principal amounts outstanding | [1] | $ 300,000,000 | 300,000,000
First Mortgage Bonds [Member] | FMB's 3.770% Due 2020 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 3.77% |
Maturity Date | [1] | 2020 |
Principal amounts outstanding | [1] | $ 100,000,000 | 100,000,000
First Mortgage Bonds [Member] | FMB's 5.300% Due 2022 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 5.30% |
Maturity Date | [1] | 2022 |
Principal amounts outstanding | [1] | $ 250,000,000 | 250,000,000
First Mortgage Bonds [Member] | FMB's 2.850% Due 2022 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 2.85% |
Maturity Date | [1] | 2022 |
Principal amounts outstanding | [1] | $ 375,000,000 | 375,000,000
First Mortgage Bonds [Member] | FMB's 3.375% Due 2023 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 3.375% |
Maturity Date | [1] | 2023 |
Principal amounts outstanding | [1] | $ 325,000,000 | 325,000,000
First Mortgage Bonds [Member] | FMB's 3.190% Due 2024 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 3.19% |
Maturity Date | [1] | 2024 |
Principal amounts outstanding | [1] | $ 52,000,000 | 52,000,000
First Mortgage Bonds [Member] | FMB's 3.125% Due August 2024 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 3.125% |
Maturity Date | [1] | 2024 |
Principal amounts outstanding | [1] | $ 250,000,000 | 250,000,000
First Mortgage Bonds [Member] | FMB's 3.390% Due 2027 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 3.39% |
Maturity Date | [1] | 2027 |
Principal amounts outstanding | [1] | $ 35,000,000 | 35,000,000
First Mortgage Bonds [Member] | FMB's 5.800% Due 2035 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 5.80% |
Maturity Date | [1] | 2035 |
Principal amounts outstanding | [1] | $ 175,000,000 | 175,000,000
First Mortgage Bonds [Member] | FMB's 6.170% Due 2040 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 6.17% |
Maturity Date | [1] | 2040 |
Principal amounts outstanding | [1] | $ 50,000,000 | 50,000,000
First Mortgage Bonds [Member] | FMB's 4.970% Due 2040 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 4.97% |
Maturity Date | [1] | 2040 |
Principal amounts outstanding | [1] | $ 50,000,000 | 50,000,000
First Mortgage Bonds [Member] | FMB's 4.310% Due 2042 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 4.31% |
Maturity Date | [1] | 2042 |
Principal amounts outstanding | [1] | $ 263,000,000 | 263,000,000
First Mortgage Bonds [Member] | FMB's 3.950% Due 2043 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 3.95% |
Maturity Date | [1] | 2043 |
Principal amounts outstanding | [1] | $ 425,000,000 | 425,000,000
First Mortgage Bonds [Member] | FMB's 4.100% Due 2045 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 4.10% |
Maturity Date | [1] | 2045 |
Principal amounts outstanding | [1] | $ 250,000,000 |
First Mortgage Bonds [Member] | FMB's 4.350% Due August 2064 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [1] | 4.35% |
Maturity Date | [1] | 2064 |
Principal amounts outstanding | [1] | $ 250,000,000 | 250,000,000
Securitization Bonds [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Principal amounts outstanding |  | $ 353,000,000 | $ 427,000,000
Securitization Bonds [Member] | Consumers Energy Company [Member] | Consumers 2014 Securitization Funding [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Weighted-average Interest Rate (%) |  | 2.69% | 2.60%
Securitization Bonds [Member] | Securitization Bonds 5.760% Due 2015 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | [2] | 5.76% |
Maturity Date |  | 2015 |
Principal amounts outstanding |  |  | $ 49,000,000
Securitization Bonds [Member] | Securitization Bonds 2.689% Due 2020-2029 [Member] | Consumers Energy Company [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate |  | 2.689% |
Maturity Date | [3] | 2020-2029 |
Principal amounts outstanding |  | $ 353,000,000 | $ 378,000,000
[1] | The weighted-average interest rate for Consumers' FMBs was 4.73 percent at December 31, 2015 and 4.75 percent at December 31, 2014.
[2] | The weighted-average interest rate for Consumers' Securitization bonds issued through its subsidiary Consumers 2014 Securitization Funding was 2.69 percent at December 31, 2015 and 2.60 percent at December 31, 2014.Principal and interest payments are made semiannually.
[3] | Principal and interest payments are made semiannually.

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