Non-Wholly Owned Entities (Details) - SouthStars Assets and Liabilities - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Current assets	$ 1,776	$ 2,886	$ 2,089
Goodwill and other intangible assets	1,926	1,952	1,957
Long-term assets and other deferred debits	10,504	10,050	9,886
Total assets	14,206	14,888	13,932
Current liabilities	2,802	3,219	2,462
Long-term liabilities and other deferred credits	7,494	7,841	7,668
Total liabilities and other deferred credits	10,296	11,060	10,130
Equity	3,910	3,828	3,802	$ 3,613
Total liabilities and equity	14,206	14,888	13,932
South Star [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Current assets	$ 202	$ 236	$ 188
Current assets (as a percent of consolidated)	11.00%	8.00%	9.00%
Goodwill and other intangible assets	$ 116	$ 125	$ 127
Goodwill and other intangible assets (as a percent of consolidated)	6.00%	6.00%	6.00%
Long-term assets and other deferred debits	$ 16	$ 17	$ 17
Long-term assets and other deferred debits (as a percent of consolidated)	0.00%	0.00%	0.00%
Total assets	$ 334	$ 378	$ 332
Total assets (as a percent of consolidated)	2.00%	3.00%	2.00%
Current liabilities	$ 45	$ 71	$ 47
Current liabilities (as a percent of consolidated)	2.00%	2.00%	2.00%
Long-term liabilities and other deferred credits	$ 1	$ 0	$ 0
Long-term liabilities and other deferred credits (as a percent of consolidated)	0.00%	0.00%	0.00%
Total liabilities and other deferred credits	$ 46	$ 71	$ 47
Total Liabilities (as a percent of consolidated)	0.00%	1.00%	0.00%
Equity	$ 288	$ 307	$ 285
Equity (as a percent of consolidated)	7.00%	8.00%	7.00%
Total liabilities and equity	$ 334	$ 378	$ 332
Total liabilities and equity (as a percent of consolidated)	2.00%	3.00%	2.00%