Fair Value of Financial Instruments and Fair Value Measurements - Valuation of Instruments (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2015 $ / MMBTU $ / MWh	Dec. 31, 2014 $ / MMBTU $ / MWh
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Instruments	Fixed price power and gas swaps with delivery period two years	Fixed price power and gas swaps with delivery period two years
Instrument Description	Transactions with delivery periods exceeding two years	Transactions with delivery periods exceeding two years
Valuation Technique	Transactions are valued against forward market prices on a discounted basis	Transactions are valued against forward market prices on a discounted basis
Valuation Inputs	Observable and extrapolated forward gas and power prices not all of which can be corroborated by market data for identical or similar products	Observable and extrapolated forward gas and power prices not all of which can be corroborated by market data for identical or similar products
SP15 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Index	SP15 ($/MWh)	SP15 ($/MWh)
Mid C [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Index	Mid C ($/MWh)	Mid C ($/MWh)
Cinergy [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Index	Cinergy ($/MWh)	Cinergy ($/MWh)
CME SWAPS MARKETS (NYMEX) [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Index	NYMEX ($/MMBtu)	NYMEX ($/MMBtu)
Weighted Average [Member] | SP15 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	47.03	43.27
Weighted Average [Member] | Mid C [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	38.12	36.16
Weighted Average [Member] | Cinergy [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	37.91	37.41
Weighted Average [Member] | CME SWAPS MARKETS (NYMEX) [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability | $ / MMBTU	4.58	4.33
Maximum [Member] | SP15 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	80.28	59.12
Maximum [Member] | Mid C [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	83.93	56.28
Maximum [Member] | Cinergy [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	77.49	68.65
Maximum [Member] | CME SWAPS MARKETS (NYMEX) [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability | $ / MMBTU	7.37	5.47
Minimum [Member] | SP15 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	23.59	30.04
Minimum [Member] | Mid C [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	11.00	12.62
Minimum [Member] | Cinergy [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability	21.17	21.17
Minimum [Member] | CME SWAPS MARKETS (NYMEX) [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Variability | $ / MMBTU	2.69	3.34