Financial Risk Management and Trading Activities - Commodity Contracts with a Fair Value (Parenthetical) (Detail) (Commodity [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total derivative contracts (Assets)	$ 309	$ 1,140
Total derivatives contract (liabilities)	312	1,177
Level 3 [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total derivative contracts (Assets)	120	494
Total derivatives contract (liabilities)	$ 15	$ 285