Financial Risk Management and Trading Activities - Changes in Physical Derivatives and Financial Assets and (Liabilities) Primarily Related to Commodities that are Measured at Fair Value Based on Level 3 Inputs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Balance at beginning of period	$ 40	$ 107	$ 212	$ 141
Unrealized pre-tax gains (losses) included in earnings	(10)	(61)	(310)	(122)
Purchases	7	4	18	44
Sales	(7)	(1)	(17)	(32)
Settlements		(7)	104	(20)
Transfers into Level 3	67	(14)	74	(12)
Transfers out of Level 3	3	(6)	19	23
Balance at end of period	$ 100	$ 22	$ 100	$ 22