Income Taxes - Additional information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Income Taxes [Line Items]
Effective tax rate of income (loss) before income taxes	32.30%	32.10%	27.80%	35.70%
Equity in net losses of unconsolidated entities	$ 9	$ 14	$ 32	$ 36
Percentage of Bonus Depreciation allowance in service before 1/1/2015					50.00%
Low-Income Housing Properties [Member]
Income Taxes [Line Items]
Equity in net losses of unconsolidated entities	5	6	17	18
Interest expense	1	1	3	4
Affordable housing tax credits and other tax benefits amount from equity method investment	10	10	25	27
Tax credits from equity method investment	7	7	17	18
Investment in Refined Coal Facility [Member]
Income Taxes [Line Items]
Equity in net losses of unconsolidated entities	2	1	4	3
Income tax benefit, including tax credits, from equity method investment	$ 7	$ 6	$ 16	$ 14