Business Segments - Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total operating revenues	$ 1,586			$ 1,699			$ 4,517	$ 5,696
Gross margin	368			391			1,179	1,228
Operating and maintenance expense	(146)			(187)			(447)	(547)
Depreciation and amortization expense	(92)			(100)			(284)	(304)
General and administrative expense	(66)			(66)			(173)	(201)
Asset Impairments	0			(247)			(746)	(247)
Other expense, net	(4)			0			(12)	(6)
Loss on sale of assets, net	0			0			0	(14)
Restructuring Costs	0			(2)			(9)	(11)
Earnings from unconsolidated affiliates	130			114			331	344
Interest expense, net	(77)			(79)			(226)	(221)
Income tax expense	1			1			2	2
Net income	112	$ 48	$ (549)	(177)	$ 120	$ 76	(389)	19
Net income attributable to noncontrolling interests	(1)			(1)			(3)	(3)
Net income attributable to partners	111			(178)			(392)	16
Non-cash derivative mark-to-market	(11)			(26)			66	(41)
Non-cash lower of cost or net realizable value adjustment							6	8
Capital expenditures	23			106			125	414
Investments in unconsolidated affiliates, net	12			56			102	326
Logistics and Marketing
Segment Reporting Information [Line Items]
Total operating revenues	1,438			1,509			3,946	5,167
Gathering and Processing
Segment Reporting Information [Line Items]
Total operating revenues	857			916			2,388	3,228
Operating Segments | Logistics and Marketing
Segment Reporting Information [Line Items]
Total operating revenues	1,438			1,509			3,946	3,228
Gross margin	88			61			268	1,021
Operating and maintenance expense	(8)			(9)			(24)	(502)
Depreciation and amortization expense	(3)			(4)			(9)	(272)
General and administrative expense	(1)			(2)			(4)	(17)
Asset Impairments				35			0	(35)
Other expense, net	(2)						(6)	(5)
Loss on sale of assets, net								(10)
Restructuring Costs				0			0	0
Earnings from unconsolidated affiliates	132			(113)			394	4
Interest expense, net	0			0			0	0
Income tax expense	0			0			0	0
Net income	206			124			619	456
Net income attributable to noncontrolling interests	0			0			0	(3)
Net income attributable to partners	206			124			619	456
Non-cash derivative mark-to-market	28			(21)			75	(26)
Capital expenditures	1			3			2	375
Investments in unconsolidated affiliates, net	12			56			102	0
Operating Segments | Gathering and Processing
Segment Reporting Information [Line Items]
Total operating revenues	857			916			2,388	5,167
Gross margin	280			330			911	207
Operating and maintenance expense	(135)			(172)			(411)	(29)
Depreciation and amortization expense	(82)			(88)			(253)	(10)
General and administrative expense	(8)			(5)			(15)	(6)
Asset Impairments				212			(746)	(212)
Other expense, net	(2)						(4)	(1)
Loss on sale of assets, net								(4)
Restructuring Costs				0			0	0
Earnings from unconsolidated affiliates	(2)			(1)			(63)	340
Interest expense, net	0			0			0	0
Income tax expense	0			0			0	0
Net income	51			(146)			(581)	13
Net income attributable to noncontrolling interests	(1)			(1)			(3)	0
Net income attributable to partners	50			(147)			(584)	10
Non-cash derivative mark-to-market	(39)			(5)			(9)	(15)
Non-cash lower of cost or net realizable value adjustment							0	0
Capital expenditures	13			99			112	28
Investments in unconsolidated affiliates, net	0			0			0	326
Other
Segment Reporting Information [Line Items]
Total operating revenues	0			0			0	0
Gross margin	0			0			0	0
Operating and maintenance expense	(3)			(6)			(12)	(16)
Depreciation and amortization expense	(7)			(8)			(22)	(22)
General and administrative expense	(57)			(59)			(154)	(178)
Asset Impairments				0			0	0
Other expense, net	0						(2)	0
Loss on sale of assets, net								0
Restructuring Costs				(2)			(9)	(11)
Earnings from unconsolidated affiliates	0			0			0	0
Interest expense, net	(77)			(79)			(226)	(221)
Income tax expense	1			1			2	2
Net income	(145)			(155)			(427)	(450)
Net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to partners	(145)			(155)			(427)	(450)
Non-cash derivative mark-to-market	0			0			0	0
Non-cash lower of cost or net realizable value adjustment							0	0
Capital expenditures	9			4			11	11
Investments in unconsolidated affiliates, net	0			0			0	0
Eliminations
Segment Reporting Information [Line Items]
Total operating revenues	(709)			(726)			(1,817)	(2,699)
Gross margin	0			0			0	0
Operating and maintenance expense	0			0				0
Depreciation and amortization expense	0			0			0	0
General and administrative expense	0			0			0	0
Asset Impairments				0				0
Other expense, net	0							0
Restructuring Costs				0			0	0
Earnings from unconsolidated affiliates	0			0			0	0
Interest expense, net	0			0			0	0
Income tax expense	0			0			0	0
Net income	0			0			0	0
Net income attributable to noncontrolling interests				0			0	0
Net income attributable to partners	0			0			0	0
Non-cash derivative mark-to-market	0			0				0
Non-cash lower of cost or net realizable value adjustment							0	0
Capital expenditures	0			0			0	0
Investments in unconsolidated affiliates, net	$ 0			$ 0			$ 0	$ 0