Retirement Benefits (Narrative) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item
Defined Benefit Plan Disclosure [Line Items]
Decrease deferred tax assets		$ (904)	$ (967)
Postretirement benefits contributions		$ 12	108	$ 262
Union employees percentage		38.00%
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions		$ 8	98	243
Union employees percentage		40.00%
DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets		$ 2,305	2,101	2,013
Period for gains or losses to be included in market related value		5 years
Postretirement benefits contributions			100
Funded status	[1]	$ (475)	(461)
Defined Benefit Plan, Accumulated Benefit Obligation		$ 2,400	2,250
DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Period for gains or losses to be included in market related value		5 years
Postretirement benefits contributions			93
Funded status		$ (455)	(441)
DB Pension Plans [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		53.00%
DB Pension Plans [Member] | Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		41.00%
DB Pension Plans [Member] | Alternative Strategy Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		6.00%
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets		$ 1,420	$ 1,264	$ 1,208
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	11 years	13 years
Estimated time of prior service cost		10 years		10 years
Funded status		$ 323	$ (144)
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets		$ 1,329	$ 1,184	$ 1,133
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	11 years	13 years
Estimated time of prior service cost		10 years		10 years
Funded status		$ 276	$ (181)
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%
Postretirement Health Trusts [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		50.00%
Postretirement Health Trusts [Member] | Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		30.00%
Postretirement Health Trusts [Member] | Alternative Strategy Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		20.00%
Postretirement Life Trusts [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		42.00%
Postretirement Life Trusts [Member] | Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		28.00%
Postretirement Life Trusts [Member] | Alternative Strategy Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations		30.00%
Defined Company Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan		$ 23	$ 20	$ 16
Defined Company Contribution Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan		22	19	16
DC SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan		$ 1
Minimum years of participation before vesting		5 years
Trust assets		$ 5	3
DC SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan		$ 1
Minimum years of participation before vesting		5 years
Trust assets		$ 5	3
401 (K) Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan cost, defined contribution plan		$ 26	24	19
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		$ 25	$ 23	$ 19
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%
Pension Plan A [Member] | DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets		$ 893
Estimated time of amortization of gains losses		9 years
Defined Benefit Plan, Accumulated Benefit Obligation		$ 1,164
Pension Plan A [Member] | DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated time of amortization of gains losses		9 years
Pension Plan B [Member] | DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated time of amortization of gains losses		20 years	10 years	10 years
Pension Plan B [Member] | DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated time of amortization of gains losses		20 years	10 years	10 years
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
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
Plan contribution percentage		15.00%
Under Age 65 [Member] | OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year		7.50%	7.00%
Under Age 65 [Member] | OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year		7.50%	7.00%
Over Age 65 [Member] | OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year		8.00%	7.75%
Over Age 65 [Member] | OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year		8.00%	7.75%

[1] At December 31, 2017, $455 million of the total funded status of the DB Pension Plans was attributable to Consumers, based on an allocation of expenses. At December 31, 2016, $441 million of the total funded status of the DB Pension Plans was attributable to Consumers, based on an allocation of expenses.