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 |