Risk Management and Trading Activities - Gross and Net Fair Values of Corporate and Energy Marketing Risk Management and Trading Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Accounts Receivable [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts designated as hedging instruments	$ 63	$ 137
Derivative contracts not designated as hedging instruments	3,185	3,216
Gross fair value of derivative contracts	3,248	3,353
Master netting arrangements	(2,621)	(2,750)
Cash collateral (received) posted	(23)	(34)
Net fair value of derivative contracts	604	569
Accounts Receivable [Member] | Commodity [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts designated as hedging instruments	23	65
Derivative contracts not designated as hedging instruments	3,163	3,188
Accounts Receivable [Member] | Interest rate and other [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts designated as hedging instruments	40	72
Accounts Receivable [Member] | Foreign exchange [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts not designated as hedging instruments		14
Accounts Receivable [Member] | Other [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts not designated as hedging instruments	22	14
Accounts Payable [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts designated as hedging instruments	(6)	(126)
Derivative contracts not designated as hedging instruments	(3,326)	(3,196)
Gross fair value of derivative contracts	(3,332)	(3,322)
Master netting arrangements	2,621	2,750
Cash collateral (received) posted	99	5
Net fair value of derivative contracts	(612)	(567)
Accounts Payable [Member] | Commodity [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts designated as hedging instruments	(5)	(124)
Derivative contracts not designated as hedging instruments	(3,315)	(3,188)
Accounts Payable [Member] | Interest rate and other [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts designated as hedging instruments	(1)	(2)
Accounts Payable [Member] | Foreign exchange [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts not designated as hedging instruments	(1)
Accounts Payable [Member] | Other [Member]
Derivatives, Fair Value [Line Items]
Derivative contracts not designated as hedging instruments	$ (10)	$ (8)