Fair Value Measurements, Recurring (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Assets:
Investments of certain benefit plans	$ 1,947	$ 1,978
Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	682	3,132
Physical purchase contracts		1
Foreign currency contracts, asset	3
Investments of certain benefit plans	75	108
Total gross fair value, assets	760	3,241
Effect of Counterparty Netting	(557)	(2,907)
Effect of Cash Collateral Netting	(12)	(99)
Commodity derivative contracts, net assets	113	126
Net Carrying Value on Balance Sheet, Assets	191	235
Cash Collateral Received Not Offset	0	0
Liabilities:
Commodity derivative contracts	557	2,920
Environmental credit obligations	2	14
Physical purchase contracts	6	5
Total gross fair value, liabilities	565	2,939
Effect of Counterparty Netting	(557)	(2,907)
Effect of Cash Collateral Netting	0	(13)
Commodity derivative contracts, net liabilities	0	0
Net Carrying Value on Balance Sheet, Liabilities	8	19
Cash Collateral Paid Not Offset	(31)	(25)
Energy Related Derivative [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Physical purchase contracts		1
Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	1,065	1,155
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	649	3,096
Physical purchase contracts		0
Foreign currency contracts, asset	3
Investments of certain benefit plans	64	97
Total gross fair value, assets	716	3,193
Liabilities:
Commodity derivative contracts	522	2,886
Environmental credit obligations	0	0
Physical purchase contracts	0	0
Total gross fair value, liabilities	522	2,886
Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	882	823
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	33	36
Physical purchase contracts		1
Foreign currency contracts, asset	0
Investments of certain benefit plans	0	0
Total gross fair value, assets	33	37
Liabilities:
Commodity derivative contracts	35	34
Environmental credit obligations	2	14
Physical purchase contracts	6	5
Total gross fair value, liabilities	43	53
Fair Value, Inputs, Level 3 [Member]
Assets:
Investments of certain benefit plans	0	0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	0	0
Physical purchase contracts		0
Foreign currency contracts, asset	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