Non-Wholly Owned Entities and Contingently Redeemable Noncontrolling Interest - SouthStar's Revenues and Expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Variable Interest Entity [Line Items]
Operating revenues	$ 571	$ 674	$ 1,905	$ 2,395
Cost of goods sold	191	222	769	1,157
Operation and maintenance	213	209	454	458
Depreciation and amortization	104	98	206	195
Taxes other than income taxes	37	38	99	114
Total operating expenses	598	567	1,584	1,924
Operating (loss) income	(27)	107	321	471
South Star [Member]
Variable Interest Entity [Line Items]
Operating revenues	118	122	372	433
Cost of goods sold	77	89	234	292
Operation and maintenance	18	18	40	41
Depreciation and amortization	2	3	4	5
Taxes other than income taxes	1	0	1	1
Total operating expenses	98	110	279	339
Operating (loss) income	$ 20	$ 12	$ 93	$ 94