Reportable Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Revenue	$ 1,670	$ 1,507	$ 1,333	$ 1,743	$ 1,620	$ 1,464	$ 1,364	$ 2,055	$ 6,253	$ 6,503	$ 6,432
Other Depreciation and amortization									598	546	576
Income (loss) from equity method investees									17	9	11
Interest Expense									389	415	431
Income Tax Expense/Benefit									245	191	224
Total Net Income Available to Common Stockholders	67	148	100	67	41	139	100	135	382	415	324
Total Plant, Property, and Equipment, Gross	15,592				14,751				15,592	14,751	14,145
Equity Method Investments	57				50				57	50	49
Total Assets	17,131				16,452				17,131	16,452	15,616
Capital Expenditures									1,266	928	883
International [Member]
Total Assets					1					1	3
Consumers Energy Company [Member]
Operating Revenue	1,608	1,448	1,282	1,675	1,565	1,397	1,303	1,988	6,013	6,253	6,156
Other Depreciation and amortization									592	542	572
Interest Expense									244	265	277
Income Tax Expense/Benefit									297	267	254
Total Net Income Available to Common Stockholders	78	162	121	76	67	154	91	153	437	465	432
Total Plant, Property, and Equipment, Gross	15,456				14,621				15,456	14,621	14,022
Total Assets	16,275				15,662				16,275	15,662	14,839
Capital Expenditures									1,261	922	877
Electric Utility [Member]
Operating Revenue									4,031	3,913	3,802
Other Depreciation and amortization									459	412	450
Interest Expense									179	192	202
Income Tax Expense/Benefit									227	190	187
Total Net Income Available to Common Stockholders									325	333	303
Total Plant, Property, and Equipment, Gross	11,041				10,400				11,041	10,400	9,944
Total Assets	10,423				9,938				10,423	9,938	9,321
Capital Expenditures									921	661	642
Electric Utility [Member] | Consumers Energy Company [Member]
Operating Revenue									4,031	3,913	3,802
Other Depreciation and amortization									459	412	450
Interest Expense									179	192	202
Income Tax Expense/Benefit									227	190	187
Total Net Income Available to Common Stockholders									325	333	303
Total Plant, Property, and Equipment, Gross	11,041				10,400				11,041	10,400	9,944
Total Assets	10,423				9,938				10,423	9,938	9,321
Capital Expenditures									921	661	642
Gas Utility [Member]
Operating Revenue									1,982	2,340	2,354
Other Depreciation and amortization									133	130	122
Interest Expense									63	71	73
Income Tax Expense/Benefit									70	77	67
Total Net Income Available to Common Stockholders									110	130	127
Total Plant, Property, and Equipment, Gross	4,400				4,206				4,400	4,206	4,063
Total Assets	5,016				4,956				5,016	4,956	4,614
Capital Expenditures									340	261	235
Gas Utility [Member] | Consumers Energy Company [Member]
Operating Revenue									1,982	2,340	2,354
Other Depreciation and amortization									133	130	122
Interest Expense									63	71	73
Income Tax Expense/Benefit									70	77	67
Total Net Income Available to Common Stockholders									110	130	127
Total Plant, Property, and Equipment, Gross	4,400				4,206				4,400	4,206	4,063
Total Assets	5,016				4,956				5,016	4,956	4,614
Capital Expenditures									340	261	235
Enterprises [Member]
Operating Revenue									183	204	238
Other Depreciation and amortization									4	3	3
Income (loss) from equity method investees									17	9	11
Income Tax Expense/Benefit									(1)	(24)	14
Total Net Income Available to Common Stockholders									16	32	36
Total Plant, Property, and Equipment, Gross	113				109				113	109	102
Equity Method Investments	55				49				55	49	48
Total Assets	181				242				181	242	191
Capital Expenditures									1	5	4
Other [Member]
Operating Revenue									57	46	38
Other Depreciation and amortization									2	1	1
Interest Expense									147	152	156
Income Tax Expense/Benefit									(51)	(52)	(44)
Total Net Income Available to Common Stockholders									(69)	(80)	(142)
Total Plant, Property, and Equipment, Gross	38				36				38	36	36
Equity Method Investments	2				1				2	1	1
Total Assets	1,511				1,316				1,511	1,316	1,490
Capital Expenditures									4	1	2
Other [Member] | Consumers Energy Company [Member]
Interest Expense									2	2	2
Total Net Income Available to Common Stockholders									2	2	2
Total Plant, Property, and Equipment, Gross	15				15				15	15	15
Total Assets	836				768				836	768	904
Maximum [Member] | International [Member]
Total Assets	$ 1								$ 1