Fair Value Measurements, Recurring (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets:
Investments of certain benefit plans	$ 71	$ 73
Total gross fair value, assets	1,100	967
Effect of Counterparty Netting	(1,013)	(893)
Effect of Cash Collateral Netting	(1)	0
Net Carrying Value on Balance Sheet, Assets	86	74
Liabilities:
Environmental credit obligations	87	104
Total gross fair value, liabilities	1,135	1,086
Effect of Counterparty Netting	(1,013)	(893)
Effect of Cash Collateral Netting	(33)	(76)
Net Carrying Value on Balance Sheet, Liabilities	89	117
Commodity Contracts [Member]
Assets:
Derivative contracts	1,026	894
Effect of Counterparty Netting	(1,013)	(893)
Effect of Cash Collateral Netting	(1)	0
Derivative contracts, net assets	12	1
Cash Collateral Received Not Offset	0	0
Liabilities:
Derivative contracts	1,046	969
Effect of Counterparty Netting	(1,013)	(893)
Effect of Cash Collateral Netting	(33)	(76)
Derivative contracts, net liabilities	0	0
Cash Collateral Paid Not Offset	(97)	(102)
Physical Purchase Contracts [Member]
Assets:
Derivative contracts	2
Derivative contracts, net assets	2
Liabilities:
Derivative contracts	2	6
Derivative contracts, net liabilities	2	6
Foreign Currency Contracts [Member]
Assets:
Derivative contracts	1
Derivative contracts, net assets	1
Liabilities:
Derivative contracts		7
Derivative contracts, net liabilities		7
Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	63	65
Total gross fair value, assets	1,075	940
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	1,030	962
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	1,011	875
Liabilities:
Derivative contracts	1,030	955
Fair Value, Inputs, Level 1 [Member] | Physical Purchase Contracts [Member]
Assets:
Derivative contracts	0
Liabilities:
Derivative contracts	0	0
Fair Value, Inputs, Level 1 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	1
Liabilities:
Derivative contracts		7
Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	0	0
Total gross fair value, assets	17	19
Liabilities:
Environmental credit obligations	87	104
Total gross fair value, liabilities	105	124
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	15	19
Liabilities:
Derivative contracts	16	14
Fair Value, Inputs, Level 2 [Member] | Physical Purchase Contracts [Member]
Assets:
Derivative contracts	2
Liabilities:
Derivative contracts	2	6
Fair Value, Inputs, Level 2 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	0
Liabilities:
Derivative contracts		0
Fair Value, Inputs, Level 3 [Member]
Assets:
Investments of certain benefit plans	8	8
Total gross fair value, assets	8	8
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	0	0
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivative contracts	0	0
Liabilities:
Derivative contracts	0	0
Fair Value, Inputs, Level 3 [Member] | Physical Purchase Contracts [Member]
Assets:
Derivative contracts	0
Liabilities:
Derivative contracts	0	0
Fair Value, Inputs, Level 3 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts	$ 0
Liabilities:
Derivative contracts		$ 0