Retirement Plans (Schedule of Allocation of Plan Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Assets for Plan Benefits	$ 1,626		$ 1,401
Investment grade bonds included in the corporate debt investments category of the plan trust assets	69.00%		61.00%
High-yield bonds included in the corporate debt investments category of the Plan trust assets	31.00%		39.00%
U.S. equity securities included in the Pooled funds category of the Plan trust assets	53.00%		55.00%
Foreign equity securities included in the Pooled funds category of the Plan trust assets	22.00%		24.00%
Foreign fixed-income securities included in the Pooled funds category of the Plan trust assets	16.00%		14.00%
U.S. fixed-income securities included in the Pooled funds category of the Plan trust assets	5.00%		4.00%
Alternative investments included in the Pooled funds category of the Plan trust assets	4.00%		3.00%
Pension Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Assets for Plan Benefits	498		431
Pension Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	1,128		970
Pension Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member]
Defined Benefit Plan, Assets for Plan Benefits	241	[1]	248	[1]
Pension Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Assets for Plan Benefits	241	[1]	248	[1]
Pension Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member]
Defined Benefit Plan, Assets for Plan Benefits	24	[2]	57	[2]
Pension Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	24	[2]	57	[2]
Pension Plans, Defined Benefit [Member] | Corporate Debt [Member]
Defined Benefit Plan, Assets for Plan Benefits	236	[3]	161	[3]
Pension Plans, Defined Benefit [Member] | Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	236	[3]	161	[3]
Pension Plans, Defined Benefit [Member] | State and Municipal Bonds [Member]
Defined Benefit Plan, Assets for Plan Benefits	10	[4]	8	[4]
Pension Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	10	[4]	8	[4]
Pension Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member]
Defined Benefit Plan, Assets for Plan Benefits	23	[5]	17	[5]
Pension Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	23	[5]	17	[5]
Pension Plans, Defined Benefit [Member] | Mutual Funds [Member]
Defined Benefit Plan, Assets for Plan Benefits	257	[6]	183	[6]
Pension Plans, Defined Benefit [Member] | Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Assets for Plan Benefits	257	[6]	183	[6]
Pension Plans, Defined Benefit [Member] | Pooled Funds [Member]
Defined Benefit Plan, Assets for Plan Benefits	835	[7]	727	[7]
Pension Plans, Defined Benefit [Member] | Pooled Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	835	[7]	727	[7]
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Assets for Plan Benefits	924		887
Investment grade bonds included in the corporate debt investments category of the plan trust assets	69.00%		61.00%
High-yield bonds included in the corporate debt investments category of the Plan trust assets	31.00%		39.00%
U.S. equity securities included in the Pooled funds category of the Plan trust assets	88.00%		89.00%
Foreign equity securities included in the Pooled funds category of the Plan trust assets	6.00%		6.00%
Foreign fixed-income securities included in the Pooled funds category of the Plan trust assets	4.00%		3.00%
U.S. fixed-income securities included in the Pooled funds category of the Plan trust assets	1.00%		1.00%
Alternative investments included in the Pooled funds category of the Plan trust assets	1.00%		1.00%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	861		825
Investment grade bonds included in the corporate debt investments category of the plan trust assets	69.00%		61.00%
High-yield bonds included in the corporate debt investments category of the Plan trust assets	31.00%		39.00%
U.S. equity securities included in the Pooled funds category of the Plan trust assets	88.00%		89.00%
Foreign equity securities included in the Pooled funds category of the Plan trust assets	6.00%		6.00%
Foreign fixed-income securities included in the Pooled funds category of the Plan trust assets	4.00%		3.00%
U.S. fixed-income securities included in the Pooled funds category of the Plan trust assets	1.00%		1.00%
Alternative investments included in the Pooled funds category of the Plan trust assets	1.00%		1.00%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Assets for Plan Benefits	208		233
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	194		216
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	716		654
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	667		609
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member]
Defined Benefit Plan, Assets for Plan Benefits	64	[1]	56	[1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	60	[1]	52	[1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Assets for Plan Benefits	64	[1]	56	[1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	60	[1]	52	[1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member]
Defined Benefit Plan, Assets for Plan Benefits	203	[2]	181	[2]
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	189	[2]	168	[2]
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	203	[2]	181	[2]
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	189	[2]	168	[2]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Corporate Debt [Member]
Defined Benefit Plan, Assets for Plan Benefits	28	[8]	20	[8]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Corporate Debt [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	26	[8]	19	[8]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	28	[8]	20	[8]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	26	[8]	19	[8]
Other Postretirement Benefit Plans, Defined Benefit [Member] | State and Municipal Bonds [Member]
Defined Benefit Plan, Assets for Plan Benefits	71	[4]	36	[4]
Other Postretirement Benefit Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	66	[4]	34	[4]
Other Postretirement Benefit Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	71	[4]	36	[4]
Other Postretirement Benefit Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	66	[4]	34	[4]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member]
Defined Benefit Plan, Assets for Plan Benefits	3	[5]	2	[5]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	3	[5]	2	[5]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	3	[5]	2	[5]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	3	[5]	2	[5]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common Stocks [Member]
Defined Benefit Plan, Assets for Plan Benefits	113	[9]	154	[9]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common Stocks [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	105	[9]	143	[9]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common Stocks [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Assets for Plan Benefits	113	[9]	154	[9]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common Stocks [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	105	[9]	143	[9]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual Funds [Member]
Defined Benefit Plan, Assets for Plan Benefits	31	[6]	23	[6]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual Funds [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	29	[6]	21	[6]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Assets for Plan Benefits	31	[6]	23	[6]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	29	[6]	21	[6]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Pooled Funds [Member]
Defined Benefit Plan, Assets for Plan Benefits	411	[10]	415	[10]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Pooled Funds [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	383	[10]	386	[10]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Pooled Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Assets for Plan Benefits	411	[10]	415	[10]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Pooled Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Assets for Plan Benefits	$ 383	[10]	$ 386	[10]

[1] Cash and short-term investments consist of money market funds with daily liquidity.

[2] U.S. government and agencies securities consist of U.S. Treasury notes and other debt securities backed by the U.S. government and related agencies. These securities were valued based on quoted market prices.

[3] At December 31, 2011, corporate debt investments in the Pension Plan comprised investment grade bonds (69 percent) and non-investment grade, high-yield bonds (31 percent) of U.S. issuers from diverse industries. At December 31, 2010, corporate debt investments in the Pension Plan comprised investment grade bonds (61 percent) and non-investment grade, high-yield bonds (39 percent) of U.S. issuers from diverse industries. These securities are valued based on quoted market prices, when available, or yields presently available on comparable securities of issuers with similar credit ratings.

[4] State and municipal bonds were valued using a matrix-pricing model that incorporates Level 2 market-based information. The fair value of the bonds was derived from various observable inputs, including benchmark yields, reported securities trades, broker/dealer quotes, bond ratings, and general information on market movements for investment grade state and municipal securities normally considered by market participants when pricing such debt securities.

[5] Foreign corporate debt securities were valued based on quoted market prices, when available, or on yields available on comparable securities of issuers with similar credit ratings.

[6] Mutual funds represent shares in registered investment companies that are priced based on the quoted NAV that is the basis for transactions to buy or sell shares in the funds.

[7] Pooled funds in the Pension Plan include both common and collective trust funds as well as special funds that contain only employee benefit plan assets from two or more unrelated benefit plans. At December 31, 2011, these funds comprised investments in U.S. equity securities (53 percent), foreign equity securities (22 percent), foreign fixed-income securities (16 percent), U.S. fixed-income securities (five percent), and alternative investments (four percent). At December 31, 2010, these funds comprised investments in U.S. equity securities (55 percent), foreign equity securities (24 percent), foreign fixed-income securities (14 percent), U.S. fixed-income securities (four percent), and alternative investments (three percent). These investments were valued at the quoted NAV provided by the fund managers that is the basis for transactions to buy or sell shares in the funds.

[8] At December 31, 2011, corporate debt investments in the OPEB plan comprised investment grade bonds (69 percent) and non-investment grade, high-yield bonds (31 percent) of U.S. issuers from diverse industries. At December 31, 2010, corporate debt investments in the OPEB plan comprised investment grade bonds (61 percent) and non-investment grade, high-yield bonds (39 percent) of U.S. issuers from diverse industries. These securities are valued based on quoted market prices, when available, or yields presently available on comparable securities of issuers with similar credit ratings.

[9] Common stocks in the OPEB plan consist of equity securities with low transaction costs that were actively managed and tracked by the S&P 500 Index. These securities were valued at their quoted closing prices.

[10] Pooled funds in the OPEB plan include both common and collective trust funds as well as special funds that contain only employee benefit plan assets from two or more unrelated benefit plans. At December 31, 2011, these funds comprised investments in U.S. equity securities (88 percent), foreign equity securities (six percent), foreign fixed-income securities (four percent), U.S. fixed-income securities (one percent), and alternative investments (one percent). At December 31, 2010, these funds comprised investments in U.S. equity securities (89 percent), foreign equity securities (six percent), foreign fixed-income securities (three percent), U.S. fixed-income securities (one percent), and alternative investments (one percent). These investments are valued at the quoted NAV provided by the fund managers that is the basis for transactions to buy or sell shares in the funds.