Financings And Capitalization (Revolving Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Revolving Credit Facilities December 21, 2017 [Member]
Expiration Date	Dec. 21, 2017	[1]
Amount of Facility	$ 550	[1]
Letters of Credit Outstanding	2	[1]
Amount Available	548	[1]
Average borrowings	12
Weighted average interest rate	2.26%
LIBOR plus	2.00%
Revolving Credit Facilities December 21, 2017 [Member] | Consumers Energy Company [Member]
Expiration Date	Dec. 21, 2017	[2]
Amount of Facility	500	[2]
Letters of Credit Outstanding	2	[2]
Amount Available	498	[2]
Revolving Credit Facilities April 18, 2017 [Member] | Consumers Energy Company [Member]
Expiration Date	Apr. 18, 2017	[2]
Amount of Facility	150	[2]
Amount Available	150	[2]
Revolving Credit Facilities September 9, 2014 [Member] | Consumers Energy Company [Member]
Expiration Date	Sep. 09, 2014	[2]
Amount of Facility	30	[2]
Letters of Credit Outstanding	$ 30	[2]

[1] Obligations under this facility are secured by Consumers common stock. CMS Energy’s average borrowings during the year ended December 31, 2012 were $12 million, with a weighted-average annual interest rate of 2.26 percent, representing LIBOR plus 2.00 percent.

[2] Obligations under this facility are secured by FMBs of Consumers.