Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	$ 103	$ 103
Reclassified losses, net of tax	2	5	$ 3
Consumers Energy Company [Member]
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	103	103
Gain on CMS Energy common stock		9
Reclassified losses, net of tax	1	4	$ 2
Mutual Fund [Member] | DB SERP [Member]
Financial Instruments [Line Items]
Reclassified losses, before tax	5
Reclassified losses, net of tax	3
Available for sale - Cost	141	152
Mutual Fund [Member] | DB SERP [Member] | Consumers Energy Company [Member]
Financial Instruments [Line Items]
Reclassified losses, before tax	4
Reclassified losses, net of tax	2
Available for sale - Cost	102	108
CMS Energy Common Stock [Member] | Consumers Energy Company [Member]
Financial Instruments [Line Items]
Available for sale - Cost	$ 4	$ 4