Non-wholly Owned Entities - SouthStars Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Current assets	$ 2,115	$ 2,886
Goodwill and other intangible assets	1,922	1,952
Long-term assets and other deferred debits	10,717	10,050
Total assets	14,754	14,888	$ 14,528
Current liabilities	3,000	3,219
Long-term liabilities and other deferred credits	7,779	7,841
Total liabilities and other deferred credits	10,779	11,060
Equity	3,975	3,828	$ 3,613	$ 3,391
Total liabilities and equity	14,754	14,888
South Star [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Current assets	$ 245	$ 236
Current assets (as a percent of consolidated)	12.00%	8.00%
Goodwill and other intangible assets	$ 114	$ 125
Goodwill and other intangible assets (as a percent of consolidated)	6.00%	6.00%
Long-term assets and other deferred debits	$ 16	$ 17
Long-term assets and other deferred debits (as a percent of consolidated)	0.00%	0.00%
Total assets	$ 375	$ 378
Total assets (as a percent of consolidated)	3.00%	3.00%
Current liabilities	$ 54	$ 71
Current liabilities (as a percent of consolidated)	2.00%	2.00%
Long-term liabilities and other deferred credits (as a percent of consolidated)	0.00%	0.00%
Long-term liabilities and other deferred credits	$ 0	$ 0
Total liabilities and other deferred credits	$ 54	$ 71
Total liabilities (as a percent of consolidated)	1.00%	1.00%
Equity	$ 321	$ 307
Equity (as a percent of consolidated)	8.00%	8.00%
Total liabilities and equity	$ 375	$ 378
Total liabilities and equity (as a percent of consolidated)	3.00%	3.00%