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		3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2013 Forecast [Member]	Sep. 30, 2013 Investment in Refined Coal Facility [Member]	Sep. 30, 2012 Investment in Refined Coal Facility [Member]	Sep. 30, 2013 Investment in Refined Coal Facility [Member]	Sep. 30, 2012 Investment in Refined Coal Facility [Member]	Sep. 30, 2013 Low-Income Housing Properties [Member]	Sep. 30, 2012 Low-Income Housing Properties [Member]	Sep. 30, 2013 Low-Income Housing Properties [Member]	Sep. 30, 2012 Low-Income Housing Properties [Member]
Income Taxes [Line Items]
Effective tax rate	34.30%	36.10%	33.60%	34.50%
Equity in net losses of unconsolidated entities	$ (3)	$ (17)	$ (19)	$ (35)		$ (1)	$ (1)	$ (4)	$ (2)	$ (6)	$ (6)	$ (18)	$ (18)
Provision for income taxes						(5)	(6)	(14)	(14)	(11)	(11)	(28)	(27)
Interest expense	119	123	363	366						2	2	5	5
Tax credits from equity method investment										$ 8	$ 8	$ 19	$ 18
Percentage of qualifying capital expenditures on property depreciated immediately					50.00%