Fair Value Measurements, Recurring (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets:
Commodity derivative contracts	$ 382	$ 3,132
Physical purchase contracts		1
Foreign currency contracts	2
Investments of certain benefit plans	78	108
Total gross fair value, assets	462	3,241
Effect of Counterparty Netting	(316)	(2,907)
Effect of Cash Collateral Netting	(11)	(99)
Commodity derivative contracts, net assets	55	126
Net Carrying Value on Balance Sheet, Assets	135	235
Cash Collateral Received Not Offset	0	0
Liabilities:
Commodity derivative contracts	316	2,920
Environmental credit obligations	4	14
Physical purchase contracts	5	5
Total gross fair value, liabilities	325	2,939
Effect of Counterparty Netting	(316)	(2,907)
Effect of Cash Collateral Netting	0	(13)
Commodity derivative contracts, net liabilities	0	0
Net Carrying Value on Balance Sheet, Liabilities	9	19
Cash Collateral Paid Not Offset	(113)	(25)
Fair Value, Inputs, Level 1 [Member]
Assets:
Commodity derivative contracts	354	3,096
Physical purchase contracts		0
Foreign currency contracts	2
Investments of certain benefit plans	67	97
Total gross fair value, assets	423	3,193
Liabilities:
Commodity derivative contracts	295	2,886
Environmental credit obligations	0	0
Physical purchase contracts	0	0
Total gross fair value, liabilities	295	2,886
Fair Value, Inputs, Level 2 [Member]
Assets:
Commodity derivative contracts	28	36
Physical purchase contracts		1
Foreign currency contracts	0
Investments of certain benefit plans	0	0
Total gross fair value, assets	28	37
Liabilities:
Commodity derivative contracts	21	34
Environmental credit obligations	4	14
Physical purchase contracts	5	5
Total gross fair value, liabilities	30	53
Fair Value, Inputs, Level 3 [Member]
Assets:
Commodity derivative contracts	0	0
Physical purchase contracts		0
Foreign currency contracts	0
Investments of certain benefit plans	11	11
Total gross fair value, assets	11	11
Liabilities:
Commodity derivative contracts	0	0
Environmental credit obligations	0	0
Physical purchase contracts	0	0
Total gross fair value, liabilities	$ 0	$ 0