Derivative Instruments and Hedging Activities - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Recurring [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Quoted Price in Active Markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets
Commodity Derivative Instruments, Liabilities
Significant Other Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets	41	3
Commodity Derivative Instruments, Liabilities		301
Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets
Commodity Derivative Instruments, Liabilities
Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets	41	3
Commodity Derivative Instruments, Liabilities		301
Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets		(2)
Commodity Derivative Instruments, Liabilities		(2)
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity Derivative Instruments, Assets	41	1
Commodity Derivative Instruments, Liabilities		$ 299