Goodwill - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Goodwill [Line Items]
Beginning balance at January 1			$ 1,858
Impairment	$ (385)	$ 0	(385)	$ 0
Ending balance at June 30, 2015	1,473		1,473
Exploration and Production [Member]
Goodwill [Line Items]
Beginning balance at January 1			1,858
Reclassification			(375)
Impairment			(385)
Ending balance at June 30, 2015	1,098		1,098
Bakken Midstream [Member]
Goodwill [Line Items]
Reclassification			375
Ending balance at June 30, 2015	$ 375		$ 375