Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2014
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	$ 103		$ 103
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	$ 4