Subsidiary Guarantors (Condensed Consolidating Statement Of Operations) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Condensed Financial Statements Captions [Line Items]
Total operating revenues	$ 567	$ 396	$ 1,179	$ 680
Total operating costs and expenses	(298)	(765)	151	(2,602)
Income (loss) from operations	269	(369)	1,330	(1,922)
Interest expense	(39)	(55)	(79)	(109)
Loss on extinguishment of debt	(1)	0	(1)	0
Other, net	16	0	16	(7)
Income (loss) before income taxes	245	(424)	1,266	(2,038)
Income tax (expense) benefit	(93)	158	(464)	752
Net income (loss)	152	(266)	802	(1,286)
Consolidation Eliminations [Member]
Condensed Financial Statements Captions [Line Items]
Total operating revenues	0	0	0	0
Total operating costs and expenses	0	0	0	0
Income (loss) from operations	0	0	0	0
Interest expense	0	0	0	0
Loss on extinguishment of debt	0		0
Other, net	(74)	73	(850)	1,713
Income (loss) before income taxes	(74)	73	(850)	1,713
Income tax (expense) benefit	0	0	0	0
Net income (loss)	(74)	73	(850)	1,713
Parent Company [Member]
Condensed Financial Statements Captions [Line Items]
Total operating revenues	0	0	0	0
Total operating costs and expenses	210	(297)	495	(218)
Income (loss) from operations	210	(297)	495	(218)
Interest expense	(38)	(54)	(78)	(107)
Loss on extinguishment of debt	(1)		(1)
Other, net	74	(73)	850	(1,713)
Income (loss) before income taxes	245	(424)	1,266	(2,038)
Income tax (expense) benefit	(93)	158	(464)	752
Net income (loss)	152	(266)	802	(1,286)
Guarantor Subsidiaries [Member]
Condensed Financial Statements Captions [Line Items]
Total operating revenues	567	396	1,179	680
Total operating costs and expenses	(508)	(468)	(344)	(2,384)
Income (loss) from operations	59	(72)	835	(1,704)
Interest expense	(1)	(1)	(1)	(2)
Loss on extinguishment of debt	0		0
Other, net	16	0	16	(7)
Income (loss) before income taxes	74	(73)	850	(1,713)
Income tax (expense) benefit	0	0	0	0
Net income (loss)	$ 74	$ (73)	$ 850	$ (1,713)