Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis Using Significant Level 3 Inputs) (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period		$ 1	$ 4	$ 2
Total gains (losses) included in earnings		(1)
Total gains (losses) offset through regulatory accounting		2	(15)	3
Purchases		1	(1)
Settlements		(2)	13	(1)
Balance at end of period		1	1	4
Unrealized gains (losses) included in earnings relating to assets and liabilities still held at end of period	[1]			(1)
Consumers Energy Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period		1	4	2
Total gains (losses) offset through regulatory accounting		2	(15)	3
Purchases			(1)
Settlements		(2)	13	(1)
Balance at end of period		$ 1	$ 1	$ 4

[1] CMS Energy records realized and unrealized gains and losses for Level 3 recurring fair value measurements in earnings as a component of operating revenue or purchased and interchange power on its consolidated statements of income.