Retirement Benefits (Schedule Of Allocation Of Plan Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,305	$ 2,101	$ 2,013
DB Pension Plans [Member] | Cash and Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21	110
DB Pension Plans [Member] | Cash and Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21	110
DB Pension Plans [Member] | U.S. Government and Agencies Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	1
DB Pension Plans [Member] | U.S. Government and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	1
DB Pension Plans [Member] | Corporate Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	336	266
DB Pension Plans [Member] | Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	336	266
DB Pension Plans [Member] | State and Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	9
DB Pension Plans [Member] | State and Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	9
DB Pension Plans [Member] | Foreign Corporate Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31	25
DB Pension Plans [Member] | Foreign Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31	25
DB Pension Plans [Member] | Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	662	571
DB Pension Plans [Member] | Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	662	571
DB Pension Plans [Member] | Plan Assets Excluding Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,063	982
DB Pension Plans [Member] | Plan Assets Excluding Pooled Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	683	681
DB Pension Plans [Member] | Plan Assets Excluding Pooled Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	380	301
DB Pension Plans [Member] | Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,242	1,119
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,420	1,264	1,208
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,329	1,184	$ 1,133
OPEB Plan [Member] | Cash and Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	39
OPEB Plan [Member] | Cash and Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	39
OPEB Plan [Member] | U.S. Government and Agencies Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1
OPEB Plan [Member] | U.S. Government and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1
OPEB Plan [Member] | Corporate Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50	38
OPEB Plan [Member] | Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50	38
OPEB Plan [Member] | State and Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
OPEB Plan [Member] | State and Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
OPEB Plan [Member] | Foreign Corporate Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	4
OPEB Plan [Member] | Foreign Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	4
OPEB Plan [Member] | Common Stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40	44
OPEB Plan [Member] | Common Stocks [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40	44
OPEB Plan [Member] | Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	647	563
OPEB Plan [Member] | Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	647	563
OPEB Plan [Member] | Plan Assets Excluding Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	759	689
OPEB Plan [Member] | Plan Assets Excluding Pooled Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	703	646
OPEB Plan [Member] | Plan Assets Excluding Pooled Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	56	43
OPEB Plan [Member] | Pooled Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 661	$ 575