Income Taxes (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended			1 Months Ended	3 Months Ended		1 Months Ended	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Apr. 30, 2010 Low-Income Housing Properties [Member]	Mar. 31, 2012 Low-Income Housing Properties [Member]	Mar. 31, 2011 Low-Income Housing Properties [Member]	Jan. 31, 2011 Investment in Refined Coal Facility [Member]	Mar. 31, 2012 Investment in Refined Coal Facility [Member]	Mar. 31, 2011 Investment in Refined Coal Facility [Member]
Schedule of Equity Method Investments [Line Items]
Cash payments to acquire equity method investment						$ 6			$ 48
Equity in net losses of unconsolidated entities	(7)	(4)					6	6
Recognized net losses resulting from entity's operating losses										less than $1 million	less than $1 million
Income tax benefit, including tax credits, from equity method investment							7	7		3	3
Interest expense	122	121					1	2
Tax credits from equity method investment							4	4
Income Taxes (Additional) (Textual) [Abstract]
Effective tax rate	32.80%	35.90%
Increase in the bonus depreciation allowance low					50.00%
Increase in the bonus depreciation allowance high					100.00%
Percentage of deduction that applies to qualifying property			50.00%	100.00%
Decrease in cash taxes due to acceleration of depreciation deduction on capital expenditures				190
Estimated decrease in cash taxes due to acceleration of depreciation deduction on current year capital expenditures			$ 90