Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012 Low-Income Housing Properties [Member]	Sep. 30, 2011 Low-Income Housing Properties [Member]	Sep. 30, 2012 Low-Income Housing Properties [Member]	Sep. 30, 2011 Low-Income Housing Properties [Member]	Dec. 31, 2012 Subsequent Event [Member]	Jan. 31, 2011 Investment in Refined Coal Facility [Member]	Sep. 30, 2012 Investment in Refined Coal Facility [Member]	Sep. 30, 2011 Investment in Refined Coal Facility [Member]	Sep. 30, 2012 Investment in Refined Coal Facility [Member]	Sep. 30, 2011 Investment in Refined Coal Facility [Member]
Schedule of Equity Method Investments [Line Items]
Effective tax rate	36.10%	32.30%	34.50%	34.00%
Cash payments to acquire equity method investment												$ 48
Equity in net losses of unconsolidated entities	(17)	(7)	(35)	(20)			(6)	(5)	(18)	(17)			(1)	(1)	(2)	(2)
Income tax benefit, including tax credits, from equity method investment							11	9	27	27			6	4	14	11
Interest expense	123	118	366	358			2	2	5	6
Tax credits from equity method investment							8	7	18	18
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					100.00%						50.00%
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