Fair Value Measurements (Assets And Liabilties Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	[1]	$ 15	$ 74
Restricted cash equivalents	[1]	26	17
Nonqualified deferred compensation plan assets	[1]	14	14
Commodity contracts	[1]		1
Total	[1]	198	252
Nonqualified deferred compensation plan liabilities	[1]	14	14
Commodity contracts	[1]		1
Total	[1]	14	15
Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted cash equivalents	[1]	26	17
Equity securities		1
Nonqualified deferred compensation plan assets	[1]	11	10
Commodity contracts	[1]		1
Total	[1]	143	155
Nonqualified deferred compensation plan liabilities	[1]	11	10
Total	[1]	11	10
DB SERP [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	[1]	6	5
DB SERP [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	[1]	5	4
DB SERP [Member] | Mutual Fund [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale - Fair Value	[1]	137	141
DB SERP [Member] | Mutual Fund [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale - Fair Value	[1]	100	102
CMS Energy Common Stock [Member] | Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	[1]	$ 1
Available for sale - Fair Value	[1]		$ 21

[1] All assets and liabilities were classified as Level 1 with the exception of some commodity contracts, which were classified as Level 3.