Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2011 percent	Dec. 31, 2010 percent	Dec. 31, 2009 percent
Reconciliation of Income Tax Expense to Federal Tax Rate [Line Items]
Income before income taxes		$ 987	$ 950	$ 782
Income tax expense at 35% statutory rate		345	333	274
Production tax credits		(6)	(33)	(12)
Investment tax credits		(6)	(6)	(7)
Depreciation		(4)	(4)	(4)
Employee Stock Ownership Plan dividends		(4)	(5)	(5)
Medicare part D subsidy		0	0	(6)
Domestic production activities deduction		(7)	(7)	(5)
Goodwill attributed to the sale of Gas Utility subsidiaries		0	0	4
Settlement of Federal tax audit		0	(12)	(11)
State and Local income taxes, net of federal benefit		37	44	25
Enactment of Michigan Corporate income Tax, net of federal expense		(87)	0	0
Other, net		(1)	1	(6)
Income Tax Expense	$ 87	$ 267	$ 311	$ 247
Effective Income Tax Rate		27.10%	32.70%	31.60%