Defined Benefit Pension And Other Postretirement Plans (Components Of Net Periodic Benefit Cost And Other Comprehensive Loss) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Other changes in plan assets and benefit obligations recognized in other comprehensive loss (pretax):
Amortization of prior service cost		$ 49	$ 50
Pension Benefits
Components of net periodic benefit cost:
Service cost		114	101		$ 88
Interest cost		73	71		74
Expected return on plan assets		(98)	(98)		(107)
Amortization – prior service credit		(46)	(46)		(46)
Amortization – actuarial loss		38	51		51
Amortization – settlement loss		7	4		96
Net periodic benefit cost	[1]	88	83		156
Other changes in plan assets and benefit obligations recognized in other comprehensive loss (pretax):
Actuarial (gain) loss		(33)	69		188
Prior service cost (credit)		0	0		0
Amortization of actuarial loss		(45)	(55)		(147)
Amortization of prior service cost		46	46		46
Other		0	0		0
Total recognized in other comprehensive loss		(32)	60		87
Total recognized in net periodic benefit cost and other comprehensive loss		56	143		243
Estimated net gain (loss) that will be amortized from accumulated other comprehensive loss in 2017		35
Estimated prior service cost that will be amortized from accumulated other comprehensive loss in 2017		39
Other Benefits
Components of net periodic benefit cost:
Service cost		32	31		27
Interest cost		35	32		33
Expected return on plan assets		0	0		0
Amortization – prior service credit		(3)	(4)		(4)
Amortization – actuarial loss		2	8		2
Amortization – settlement loss		0	0		0
Net periodic benefit cost	[1]	66	67		58
Other changes in plan assets and benefit obligations recognized in other comprehensive loss (pretax):
Actuarial (gain) loss		(101)	(63)		86
Prior service cost (credit)		0	13	[2]	0
Amortization of actuarial loss		(2)	(8)		(2)
Amortization of prior service cost		3	4		4
Other		0	0		0
Total recognized in other comprehensive loss		(100)	(54)		88
Total recognized in net periodic benefit cost and other comprehensive loss		(34)	$ 13		$ 146
Estimated net gain (loss) that will be amortized from accumulated other comprehensive loss in 2017		2
Estimated prior service cost that will be amortized from accumulated other comprehensive loss in 2017		$ 3

[1] Net periodic benefit cost reflects a calculated market-related value of plan assets which recognizes changes in fair value over three years.

[2] Includes adjustments related to the MarkWest Merger in 2015