Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Income Tax Expense to Federal Tax Rate [Line Items]
Income before income taxes		$ 960	$ 991	$ 962
Income tax expense at 35% statutory rate		336	347	337
Production tax credits		(49)	(6)	(33)
Investment tax credits		(6)	(6)	(6)
Depreciation		(4)	(4)	(4)
Employee Stock Ownership Plan dividends		(4)	(4)	(5)
Domestic production activities deduction		(14)	(7)	(7)
Settlement of Federal tax audit		0	0	(12)
State and Local income taxes, net of federal benefit		257	267	311
State and local income taxes, net of federal benefit		37	37	44
Enactment of Michigan Corporate income Tax, net of federal expense		0	(87)	0
Other, net		(10)	(2)	1
Income Tax Expense	$ 87	$ 286	$ 268	$ 315
Effective Income Tax Rate		29.80%	27.00%	32.70%