Retirement Benefits (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions	$ 108	$ 262	$ 32
Union employees percentage	39.00%
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions	$ 98	243	29
Union employees percentage	41.00%
DB Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 2,101	$ 2,013	$ 1,979
Estimated time of amortization of gains losses	10 years	10 years	10 years
Period for gains or losses to be included in market related value	5 years
Postretirement benefits contributions	$ 100	$ 225
DB Pension Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated time of amortization of gains losses	10 years	10 years	10 years
Period for gains or losses to be included in market related value	5 years
Postretirement benefits contributions	$ 93	$ 209
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 1,264	$ 1,208	$ 1,265
Retirement age requirement | item	55	55	55
Retirement years of service	10 years
Retirement years of service with disability	15 years
Ultimate health care cost trend rate	4.75%
Year that rate reaches ultimate trend rate	2027
Estimated time of amortization of gains losses	11 years	13 years	13 years
Estimated time of prior service cost		10 years
Postretirement benefits contributions		$ 29	$ 25
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 1,184	$ 1,133	$ 1,186
Retirement age requirement | item	55	55	55
Retirement years of service	10 years
Retirement years of service with disability	15 years
Ultimate health care cost trend rate	4.75%
Year that rate reaches ultimate trend rate	2027
Estimated time of amortization of gains losses	11 years	13 years	13 years
Estimated time of prior service cost		10 years
Postretirement benefits contributions		$ 29	$ 25
Pension And OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortized net gains and losses in excess of PBO or MRV	10.00%	10.00%	10.00%
Pension And OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortized net gains and losses in excess of PBO or MRV	10.00%	10.00%	10.00%
Defined Company Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan	$ 20	$ 16	$ 13
Defined Company Contribution Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan	$ 19	16	13
DC SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum years of participation before vesting	5 years
Trust assets	$ 3	2
DC SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum years of participation before vesting	5 years
Trust assets	$ 3	2
401 (K) Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan	$ 24	19	18
Employer match of eligible contributions	100.00%
Employer match of eligible wages	3.00%
Secondary employer match of eligible contributions	50.00%
Secondary employer match of eligible wages	2.00%
401 (K) Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan	$ 23	19	18
Employer match of eligible contributions	100.00%
Employer match of eligible wages	3.00%
Secondary employer match of eligible contributions	50.00%
Secondary employer match of eligible wages	2.00%
Minimum [Member] | Defined Company Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer match of eligible wages	5.00%
Minimum [Member] | Defined Company Contribution Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer match of eligible wages	5.00%
Minimum [Member] | DC SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan contribution percentage	5.00%
Minimum [Member] | DC SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan contribution percentage	5.00%
Maximum [Member] | Defined Company Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer match of eligible wages	7.00%
Maximum [Member] | Defined Company Contribution Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer match of eligible wages	7.00%
Maximum [Member] | DC SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan	$ 1	1	1
Plan contribution percentage	15.00%
Maximum [Member] | DC SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan	$ 1	$ 1	$ 1
Plan contribution percentage	15.00%
Under Age 65 [Member] | OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year	7.00%	7.25%
Under Age 65 [Member] | OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year	7.00%	7.25%
Over Age 65 [Member] | OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year	7.75%	8.00%
Over Age 65 [Member] | OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year	7.75%	8.00%