Fair Value Measurements, Recurring (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets:
Investments of certain benefit plans	$ 2,428	$ 2,097
Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	1,078	1,115
Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	1,350	982
Fair Value, Inputs, Level 3 [Member]
Assets:
Investments of certain benefit plans	0	0
Fair Value, Measurements, Recurring [Member]
Assets:
Investments of certain benefit plans	73	69
Total gross fair value, assets	967	984
Effect of Counter- party Netting	(893)	(875)
Effect of Cash Collateral Netting	0	0
Net Carrying Value on Balance Sheet, Assets	74	109
Liabilities:
Environmental credit obligations	104	188
Total gross fair value, liabilities	1,086	1,088
Effect of Counter- party Netting	(893)	(875)
Effect of Cash Collateral Netting	(76)	(20)
Net Carrying Value on Balance Sheet, Liabilities	117	193
Fair Value, Measurements, Recurring [Member] | Commodity Derivative Contracts [Member]
Assets:
Derivative contracts	894	912
Effect of Counter- party Netting	(893)	(875)
Effect of Cash Collateral Netting	0	0
Derivative contracts, net assets	1	37
Cash Collateral Received Not Offset	0	0
Liabilities:
Derivative contracts	969	895
Effect of Counter- party Netting	(893)	(875)
Effect of Cash Collateral Netting	(76)	(20)
Derivative contracts, net liabilities	0	0
Cash Collateral Paid Not Offset	(102)	(88)
Fair Value, Measurements, Recurring [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	6	5
Derivative contracts, net liabilities	6	5
Fair Value, Measurements, Recurring [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts		3
Derivative contracts, net assets		3
Liabilities:
Derivative contracts	7
Derivative contracts, net liabilities	7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	65	58
Total gross fair value, assets	940	935
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	962	872
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commodity Derivative Contracts [Member]
Assets:
Derivative contracts	875	874
Liabilities:
Derivative contracts	955	872
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts		3
Liabilities:
Derivative contracts	7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	0	0
Total gross fair value, assets	19	38
Liabilities:
Environmental credit obligations	104	188
Total gross fair value, liabilities	124	216
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commodity Derivative Contracts [Member]
Assets:
Derivative contracts	19	38
Liabilities:
Derivative contracts	14	23
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	6	5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts		0
Liabilities:
Derivative contracts	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Investments of certain benefit plans	8	11
Total gross fair value, assets	8	11
Liabilities:
Environmental credit obligations	0	0
Total gross fair value, liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Derivative Contracts [Member]
Assets:
Derivative contracts	0	0
Liabilities:
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Physical Purchase Contracts [Member]
Liabilities:
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Currency Contracts [Member]
Assets:
Derivative contracts		$ 0
Liabilities:
Derivative contracts	$ 0