Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 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 donation of CMS Energy common stock	$ 4