Summary Of Significant Accounting Policies (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Disclosure Summary Of Significant Accounting Policies Narrative [Abstract]
Fees related to operation of jointly owned oil and natural gas properties	$ 4	$ 4	$ 8	$ 8
ASU 2016-09 Cumulative Effect: Forfeiture estimate compensation expense / increase to APIC			8
ASU 2016-09 Cumulative Effect: Deferred tax benefit			3
ASU 2016-09 Cumulative Effect: Excess tax benefits			5
ASU 2016-09 Cumulative Effect: Decrease to retained earnings			0
ASU 2016-09 Cumulative Effect: Decrease to deferred income taxes			8
Excess tax benefit (deficiency) [discrete item]	(2)		6
Forfeitures expense	1		6
Alpha Crude Connector [Member]
Equity Method Investments [Line Items]
Total equity method investment					$ 129
Equity method investment ownership percentage					50.00%
Other [Member]
Equity Method Investments [Line Items]
Total equity method investment	$ 45		$ 45		$ 42
Equity method investment ownership percentage	23.75%		23.75%