Risk Management and Hedging Activities - Schedule of Changes in Derivative Instruments not Designated as Hedging Instruments (Detail) - Derivative Assets Not Designated As Hedging Instruments [Member] - Commodity Derivatives [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Affiliates [Member]
Derivative [Line Items]
Realized	$ 14	$ 54
Unrealized	(9)	(40)
(Losses) gains from commodity derivative activity, net	5	14
Third Party [Member]
Derivative [Line Items]
Realized	40	7
Unrealized	(36)	(2)
(Losses) gains from commodity derivative activity, net	$ 4	$ 5