Summary Of Significant Accounting Policies (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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
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 (discrete item)	7
Forfeitures expense	5
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	$ 43		$ 42
Equity method investment ownership percentage	23.75%